[ELF] Allow placing SHF_MERGE sections with different alignments into the same...

[ELF] Allow placing SHF_MERGE sections with different alignments into the same MergeSyntheticSection

This should fix PR42289: the Linux kernel has a use case that input
files have .rodata.cst32 sections with different alignments. The
expectation (and what ld.bfd and gold do) is that in the -r link, there
is only one .rodata.cst32 (SHF_MERGE sections with different alignments
can be combined), but lld currently creates one for each different
alignment.

The current merging strategy:

1) Group SHF_MERGE sections by (name, sh_flags, sh_entsize and
   sh_addralign). String merging is performed among a group, even if -O0 is specified.
2) Create one output section for each group. This is a special case in
   addInputSec().

This patch changes 1) to:

1) Group SHF_MERGE sections by (name, sh_flags, sh_entsize).
   String merging is performed among a group, even if -O0 is specified.

We will thus create just one .rodata.cst32 . This also improves merging
efficiency when sections with the same name but different alignments are
combined.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D63432

llvm-svn: 365015