Thread safety analysis: Mock getter for private mutexes can be undefined

Thread safety analysis: Mock getter for private mutexes can be undefined

Usage in an annotation is no odr-use, so I think there needs to be no
definition. Upside is that in practice one will get linker errors if it
is actually odr-used instead of calling a function that returns 0.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D106375