[profile] Mark lprofCurFilename as COMPILER_RT_WEAK

This makes __llvm_profile_set_filename() work across dylib boundaries on
Darwin.

This functionality was originally meant to work on all platforms, but
was moved to a Linux-only directory with r272404. The root cause of the
test failure on Darwin was that lprofCurFilename was not marked weak.
Each dylib maintained its own copy of the variable due to the two-level
namespace.

Tested with check-profile (on Darwin). I don't expect this to regress
other platforms.

Differential Revision: https://reviews.llvm.org/D25707

llvm-svn: 284440