Add support for non-contiguous blocks to find_pc_partial_function

Add support for non-contiguous blocks to find_pc_partial_function

This change adds an optional output parameter BLOCK to
find_pc_partial_function.  If BLOCK is non-null, then *BLOCK will be
set to the address of the block corresponding to the function symbol
if such a symbol was found during lookup.  Otherwise it's set to the
NULL value.  Callers may wish to use the block information to
determine whether the block contains any non-contiguous ranges.  The
caller may also iterate over or examine those ranges.

When I first started looking at the broken stepping behavior associated
with functions w/ non-contiguous ranges, I found that I could "fix"
the problem by disabling the find_pc_partial_function cache.  It would
sometimes happen that the PC passed in would be between the low and
high cache values, but would be in some other function that happens to
be placed in between the ranges for the cached function.  This caused
incorrect values to be returned.

So dealing with this cache turns out to be very important for fixing
this problem.  I explored three different ways of dealing with the
cache.

My first approach was to clear the cache when a block was encountered
with more than one range.  This would cause the non-cache pathway to
be executed on the next call to find_pc_partial_function.

Another approach, which I suspect is slightly faster, checks to see
whether the PC is within one of the ranges associated with the cached
block.  If so, then the cached values can be used.  It falls back to
the original behavior if there is no cached block.

The current approach, suggested by Simon Marchi, is to restrict the
low/high pc values recorded for the cache to the beginning and end of
the range containing the PC value under consideration.  This allows us
to retain the simple (and fast) test for determining whether the
memoized (cached) values apply to the PC passed to
find_pc_partial_function.

Another choice that had to be made regards setting *ADDRESS and
*ENDADDR.  There are three possibilities which might make sense:

1) *ADDRESS and *ENDADDR represent the lowest and highest address
   of the function.
2) *ADDRESS and *ENDADDR are set to the start and end address of
   the range containing the entry pc.
3) *ADDRESS and *ENDADDR are set to the start and end address of
   the range in which PC is found.

An earlier version of this patch implemented option #1.  I found out
that it's not very useful though and, in fact, returns results that
are incorrect when used in the context of determining the start and
end of the function for doing prologue analysis.  While debugging a
function in which the entry pc was in the second range (of a function
containing two non-contiguous ranges), I noticed that
amd64_skip_prologue called find_pc_partial_function - the returned
start address was set to the beginning of the first range.  This is
incorrect for this function.  What was also interesting was that this
first invocation of find_pc_partial_function correctly set the cache
for the PC on which it had been invoked, but a slightly later call
from skip_prologue_using_sal could not use this cached value because
it was now being used to lookup the very lowest address of the
function - which is in a range not containing the entry pc.

Option #2 is attractive as it would provide a desirable result
when used in the context of prologue analysis.  However, many callers,
including some which do prologue analysis want the condition
*ADDRESS <= PC < *ENDADDR to hold.  This will not be the case when
find_pc_partial_function is called on a PC that's in a non-entry-pc
range.  A later patch to this series adds
find_function_entry_range_from_pc as a wrapper of
find_pc_partial_function.

Option #3 causes the *ADDRESS <= PC < *ENDADDR property to hold.  If
find_pc_partial_function is called with a PC that's within entry pc's
range, then it will correctly return the limits of that range.  So, if
the result of a minsym search is passed to find_pc_partial_function
to find the limits, then correct results will be achieved.  Returned
limits (for prologue analysis) won't be correct when PC is within some
other (non-entry-pc) range.  I don't yet know how big of a problem
this might be; I'm guessing that it won't be a serious problem - if a
compiler generates functions which have non-contiguous ranges, then it
also probably generates DWARF2 CFI which makes a lot of the old
prologue analysis moot.

I've implemented option #3 for this version of the patch.  I don't see
any regressions for x86-64.  Moreover, I don't expect to see
regressions for other targets either simply because
find_pc_partial_function behaves the same as it did before for the
contiguous address range case.  That said, there may be some
adjustments needed if GDB encounters a function requiring prologue
analysis which occupies non-contiguous ranges.

gdb/ChangeLog:

	* symtab.h (find_pc_partial_function): Add new parameter `block'.
	* blockframe.c (cache_pc_function_block): New static global.
	(clear_pc_function_cache): Clear cache_pc_function_block.
	(find_pc_partial_function): Move comment to symtab.h.  Add
	support for non-contiguous blocks.