During LTO, we call 'dsymutil' when we compile source files. This necessitates

clang specifying a temporary file that it later cleans up so that it can survive
the linking stage. However, when we compile object files during LTO we don't
call 'dsymutil'. That's done at a different stage (if at all). We rely upon the
linker to specify a unique name for the temporary file it generates.
<rdar://problem/12401423>

llvm-svn: 165028