Replace separator in OpenMP variant name mangling.

Summary:
Nvidia PTX does not allow `.` to appear in identifiers, so OpenMP variant mangling now uses `$` to separate segments of the mangled name for variants of functions declared via `declare variant`.

Reviewers: jdoerfert, Hahnfeld

Reviewed By: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, cfe-commits

Tags: #openmp, #clang

Differential Revision: https://reviews.llvm.org/D80439