[cfi] Cross-DSO CFI diagnostic mode (compiler-rt part)

* add __cfi_slowpath_diag with a 3rd parameter which is a pointer to
  the diagnostic info for the ubsan handlers.
*__cfi_check gets a 3rd parameter as well.
* unify vcall/cast/etc and icall diagnostic info format, and merge
  the handlers to have a single entry point (actually two points due
  to abort/noabort variants).
* tests

Note that this comes with a tiny overhead in the non-diag mode:
cfi_slowpath must pass 0 as the 3rd argument to cfi_check.

llvm-svn: 258744