[clang-tidy] Don't warn about call to unresolved operator*

Summary:
The misc-unconventional-assign-operator check had a false positive
warning when the 'operator*' in 'return *this' was unresolved.

Change matcher to allow calls to unresolved operator.

Fixes PR31531.

Reviewers: alexfh, aaron.ballman

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D29393

llvm-svn: 295176