[profile] Move __llvm_profile_filename into a separate object

Users can specify the path a raw profile is written to by passing
-fprofile-instr-generate=<path>, but this functionality broke on Darwin
after __llvm_profile_filename was made weak [1], resulting in profiles
being written to "default.profraw" even when <path> is specified.

The situation is that instrumented programs provide a weak definition of
__llvm_profile_filename, which conflicts with a weak redefinition
provided by the profiling runtime.

The linker appears to pick the 'winning' definition arbitrarily: on
Darwin, it usually prefers the larger definition, which is probably why
the instrprof-override-filename.c test has been passing.

The fix is to move the runtime's definition into a separate object file
within the archive. This means that the linker won't "see" the runtime's
definition unless the user program has not provided one. I couldn't
think of a great way to test this other than to mimic the Darwin
failure: use -fprofile-instr-generate=<some-small-path>.

Testing: check-{clang,profile}, modified instrprof-override-filename.c.

[1] [Profile] deprecate __llvm_profile_override_default_filename
https://reviews.llvm.org/D22613
https://reviews.llvm.org/D22614

Differential Revision: https://reviews.llvm.org/D34797

llvm-svn: 306710