Add GCC-compatible flags -fprofile-generate and -fprofile-use.

This patch adds support for specifying where the profile is emitted in a
way similar to GCC. These flags are used to specify directories instead
of filenames. When -fprofile-generate=DIR is used, the compiler will
generate code to write to <DIR>/default.profraw.

The patch also adds a couple of extensions: LLVM_PROFILE_FILE can still be
used to override the directory and file name to use and -fprofile-use
accepts both directories and filenames.

To simplify the set of flags used in the backend, all the flags get
canonicalized to -fprofile-instr-{generate,use} when passed to the
backend. The decision to use a default name for the profile is done
in the driver.

llvm-svn: 241825