Reland integrates build fixes & further review suggestions.
Thanks to @zturner for the initial S_OBJNAME patch!
Differential Revision: https://reviews.llvm.org/D43002
Also revert all subsequent fixes:
- abd1cbf5e543f0f114d2742e109ead7d7ddbf9c4 [Clang] Disable debug-info-objname.cpp test on Unix until I sort out the issue.
- 00ec441253048f5e30540ea26bb0a28c42a5fc18 [Clang] debug-info-objname.cpp test: explictly encode a x86 target when using %clang_cl to avoid falling back to a native CPU triple.
- cd407f6e52b09cce2bef24c74b7f36fedc94991b [Clang] Fix build by restricting debug-info-objname.cpp test to x86.