[clang] Fix CodeGenAction for LLVM IR MemBuffers

Replaces use of getCurrentFile with getCurrentFileOrBufferName
in CodeGenAction. This avoids an assertion error or an incorrect
name chosen for the output file when assertions are disabled.
This error previously occurred when the FrontendInputFile was a
MemoryBuffer instead of a file.

Reviewed By: jlebar

Differential Revision: https://reviews.llvm.org/D121259