Make expand_location resolve to locus in main source file

Make expand_location resolve to locus in main source file

Apparently, quite some places in the compiler (like the C/C++
preprocessor, the debug info machinery) expect expand_location to
resolve to locations that are in the main source file, even if the
token at stake comes from a macro that was defined in a header
somewhere.  Turning on -ftrack-macro-expansion by default was
triggering a lot of failures (not necessarily related to diagnostics)
because expand_location resolves to spelling locations instead.

So I have changed expand_location to honour the initial expectation.

In addition, I came up with the new expand_location_to_spelling_point
used in diagnostic_build_prefix because the diagnostic system, on the
other hand, wants to point to the location of the token where it was
spelled, and then display the error context involving all the macro
whose expansion led to that spelling point - if we are in the context
of a macro expansion there.

This seems to me like a reasonable balance.

Tested and bootstrapped on x86_64-unknown-linux-gnu against trunk and
whitnessed that a lot more tests were PASSing.

Note that the bootstrap with -ftrack-macro-expansion exhibits other
separate issues that are addressed in subsequent patches.  This patch
just fixes one class of problems.

The patch does pass bootstrap with -ftrack-macro-expansion turned off,
though.

gcc/

	* input.c (expand_location_1): New.  Takes a parameter to choose
	whether to resolve the location to spelling or expansion point.
	Was factorized from ...
	(expand_location): ... here.
	(expand_location_to_spelling_point): New.  Implemented in terms of
	expand_location_1.
	* diagnostic.c (diagnostic_build_prefix): Use the new
	expand_location_to_spelling_point instead of expand_location.

From-SVN: r186969