[ELF] Resolve relocations in .debug_* referencing (discarded symbols or ICF folded...

[ELF] Resolve relocations in .debug_* referencing (discarded symbols or ICF folded section symbols) to tombstone values

See D59553, https://lists.llvm.org/pipermail/llvm-dev/2020-May/141885.html and
https://sourceware.org/pipermail/binutils/2020-May/111357.html for
extensive discussions on a tombstone value.
See http://www.dwarfstd.org/ShowIssue.php?issue=200609.1
(Reserve an address value for "not present") for a DWARF enhancement proposal.

We resolve such relocations to a tombstone value to indicate that the address is invalid.
This solves several problems (the normal behavior is to resolve the relocation to the addend):

* For an empty function in a collected section, a pair of (0,0) can
  terminate .debug_loc and .debug_ranges (as of binutils 2.34, GNU ld
  resolves such a relocation to 1 to avoid the .debug_ranges issue)
* If DW_AT_high_pc is sufficiently large, the address range can collide
  with a regular code range of low address (https://bugs.llvm.org/show_bug.cgi?id=41124 )
* If a text section is folded into another by ICF, we may leave entries
  in multiple CUs claiming ownership of the same range of code, which can
  confuse consumers.
* Debug information associated with COMDAT sections can have problems
  similar to ICF, but is more complex - thus not addressed by this patch.

For pre-DWARF-v5 .debug_loc and .debug_ranges, a pair of 0 can terminate
entries (invalidating subsequent ranges).
-1 is a reserved value with special meaning (base address selection entry) which can't be used either.
Use -2 instead.

For all other .debug_*, use UINT32_MAX for 32-bit targets and UINT64_MAX
for 64-bit targets. In the code, we intentionally use
`uint64_t tombstone = UINT64_MAX` for 32-bit targets as well: this matches
SignExtend64 as used in `relocateAlloc`. (Actually UINT32_MAX does not work for R_386_32)

Note 0, we only special case `target->symbolicRel` (R_X86_64_64, R_AARCH64_ABS64, R_PPC64_ADDR64), not
short-range absolute relocations (e.g. R_X86_64_32). Only forms like DW_FORM_addr need to be special cased.
They can hold an arbitrary address (must be 64-bit on a 64-bit target). (In theory,
producers can make use of small code model to emit 32-bit relocations. This doesn't seem to be leveraged.)

Note 1, we have to ignore the addend, because we don't want to resolve
DW_AT_low_pc (which may have a non-zero addend) to -1+addend (wrap
around to a low address):

  __attribute__((section(".text.x"))) void f1() { }
  __attribute__((section(".text.x"))) void f2() { } // DW_AT_low_pc has a non-zero addend

Note 2, if the prevailing copy does not have debugging information while
a non-prevailing copy has (partial debug build), we don't do extra work
to attach debugging information to the prevailing definition.  (clang
has a lot of debug info optimizations that are on-by-default that assume
the whole program is built with debug info).

  clang -c -ffunction-sections a.cc    # prevailing copy has no debug info
  clang -c -ffunction-sections -g b.cc

Reviewed By: dblaikie, avl, jhenderson

Differential Revision: https://reviews.llvm.org/D81784