[XRay][compiler-rt] Merge XRay FDR mode into a single file (NFC)
We planned to have FDR mode's internals unit-tested but it turns out
that we can just use end-to-end testing to verify the implementation.
We're going to move towards that approach more and more going forward,
so we're merging the implementation details of FDR mode into a single
We also avoid globbing in the XRay test helper macro, and instead list
down the files from the lib directory.