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
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 434dc4d..20a308d 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -379,6 +379,7 @@
                    false) ||
       Args.hasArg(options::OPT_fprofile_generate) ||
       Args.hasArg(options::OPT_fprofile_instr_generate) ||
+      Args.hasArg(options::OPT_fprofile_instr_generate_EQ) ||
       Args.hasArg(options::OPT_fcreate_profile) ||
       Args.hasArg(options::OPT_coverage)) {
     // Select the appropriate runtime library for the target.