[clang-tidy] Do not issue fix for functions that are referenced outside of callExpr

Summary: Suppress fixes for functions that are referenced within the
compilation unit outside of a call expression as the signature change
could break the code referencing the function.

We still issue a warning in this case so that users can decide to
manually change the function signature.

Reviewers: alexfh, sbenza, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26203

llvm-svn: 286424