InstrProf: Support for setting profile output from command line

This change is the third 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 clang frontend, and adds a
new test.

The compiler-rt and llvm parts are r236055 and r236288, respectively.

Patch by Teresa Johnson. Thanks!

llvm-svn: 236289