InstrProf: Instrumenter support for setting profile output from command line

InstrProf: Instrumenter support for setting profile output from command line

This change is the second of 3 patches to add support for specifying
the profile output from the command line via -fprofile-instr-generate=<path>,
where the specified output path/file will be overridden by the
LLVM_PROFILE_FILE environment variable.

This patch adds the necessary support to the llvm instrumenter, specifically
a new member of GCOVOptions for clang to save the specified filename, and
support for calling the new compiler-rt interface from __llvm_profile_init.

Patch by Teresa Johnson. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236288 91177308-0d34-0410-b5e6-96231b3b80d8