The function sanitizer only checks indirect calls through function
pointers. This excludes all non-static member functions (constructor
calls, calls through thunks, etc all use a separate code path). Don't
emit function signatures for functions that won't be checked.
Apart from cutting down on code size, this should fix a regression on
Linux caused by r313096. For context, see the mailing list discussion:
r313096 - [ubsan] Function Sanitizer: Don't require writable text segments
Testing: check-clang, check-ubsan