[ubsan] Don't emit function signatures for non-static member functions

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

Differential Revision: https://reviews.llvm.org/D38913

llvm-svn: 315786