[llvm-profgen] Fix perf script parsing issues

Fix two perf script parsing issues:

1) Redirect the error message to a new file. (the error message mixed in the perfscript could screw up the MMAP event line and cause a parsing failure)

2) Changed the MMap parsing error message to warning since the perfscript can still be parsed using the preferred address as base address.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D131449