[MemProf] Allow the binary to specify the profile output filename
This will allow the output directory to be specified by a build time
option, similar to the directory specified for regular PGO profiles via
-fprofile-generate=. The memory profiling instrumentation pass will
set up the variable. This is the same mechanism used by the PGO
instrumentation and runtime.
Differential Revision: https://reviews.llvm.org/D89086