[PATCH] Pass -mglobal-merge as a module flag metadata.

[Ahmed Bougacha]

Hi dexonsmith,

Per title, we used to pass -mno-global-merge when specified, and enable GlobalMerge always (leaving it to LLVM to decide whether to actually enable it: it does, on ARM and AArch64, for -O1 and above).

This addresses two problems (I'm fine with splitting the patch if desired):
- with LTO, passing -mno-global-merge didn't do anything, since LTO implies -O3 from the backend's standpoint.
- GlobalMerge was enabled by -O1.  Instead, on AArch64, enable it at -O3, or when -mglobal-merge is specified. (the goal is to have it predicated by -O3 on ARM as well, but I still need to measure before doing that.)

Now, -mglobal-merge and -mno-global-merge are both explicitly passed by the driver.
They are then passed to the backend using module flag metadata (bikeshedding time: name is prose, and value is string.  We can change both, and AFAICT there's no similar backend flag to be consistent with, yet.)

Thanks!
-Ahmed

http://reviews.llvm.org/D7968

Files:
  include/clang/Driver/Options.td
  include/clang/Frontend/CodeGenOptions.def
  lib/CodeGen/BackendUtil.cpp
  lib/CodeGen/CodeGenModule.cpp
  lib/CodeGen/CodeGenModule.h
  lib/Driver/Tools.cpp
  lib/Frontend/CompilerInvocation.cpp
  test/CodeGen/global-merge.c
  test/Driver/mglobal-merge.c
  test/Driver/mno-global-merge.c

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