[trace][intelpt] Update TraceIntelPTBundleSaver.cpp to accommodate FileSpec API changes

D130309 introduced changes to the FileSpec API which broke usages of
`GetCString()` in TraceIntelPTBundleSaver.cpp. This diff replaces usages
of `GetCString()` with `GetPath().c_str()` as suggested by D130309.

Test Plan:
Building with the trace plug-in now succeeds

Differential Revision: https://reviews.llvm.org/D130924