Core: preprocess out TestPromptFormats

TestPromptFormats appears as though it may be a useful unit test.
Unfortunately, there is no invocation mechanism in place right now.  It is
unclear how to add a unit test for this scenario to the existing tests.  It
would be ideal to remove this entirely, but I am hopeful that this can/will be
pulled out into a test still since it uses a user accessible interface.
cleanup unreferenced functions

This is a mechanical cleanup of unused functions.  In the case where the
functions are referenced (in comment form), I've simply commented out the
functions.  A second pass to clean that up is warranted.

The functions which are otherwise unused have been removed.  Some of these were
introduced in the initial commit and not in use prior to that point!

NFC
cmake: fix build with CompilerRT

A redefinition of append_if in CompilerRTUtils causes the append_if to fail.
Manually append the values to the flags.

Thanks to Kuba Ober for pointing this out!