[profile] Do not cache __llvm_profile_get_filename result

When the %m filename pattern is used, the filename is unique to each
image, so the cached value is wrong.

It struck me that the full filename isn't something that's recomputed
often, so perhaps it doesn't need to be cached at all. David Li pointed
out we can go further and just hide lprofCurFilename. This may regress
workflows that depend on using the set-filename API to change filenames
across all loaded DSOs, but this is expected to be very rare.

rdar://55137071

Differential Revision: https://reviews.llvm.org/D69137

llvm-svn: 375301