swift234

Teach the Swift front-end to generate code with
"Sanitizer Coverage" with a new flag ``-sanitize-coverage=``. This
flag is analogous to Clang's ``-fsanitize-coverage=``.

This instrumentation currently requires ASan or TSan to be enabled
because the module pass created by ``createSanitizerCoverageModulePass()``
inserts calls into functions found in compiler-rt's "sanitizer_common".
"sanitizer_common" is not shipped as an individual library but instead
exists in several of the sanitizer runtime libraries so we have to
link with one of them to avoid linking errors.

The rationale between adding this feature is to allow experimentation
with libFuzzer which currently relies on "Sanitizer Coverage"
instrumentation.