[llvm-gsymutil] Fix tracking of currently open file

Prior to this change, `CurrentGSYMPath` was never updated. As a consequence, the GSYM file was reopened for every frame, even if all frames were relative to the same GSYM file.

This change brings a 13x speedup on a test I'm doing (symbolizing ~25K frames from libxul)

(This is my first-ever LLVM change - sorry if I missed something in the process!)

Reviewed By: simon.giesecke, clayborg

Differential Revision: https://reviews.llvm.org/D132912