[ASTImporter] Remove redundant IsStructuralMatch overloads

Nearly all of the overloads have pretty much the same behaviour. The only
exception here is that some of them call back `GetOriginalDecl` and others
don't, but the only real user of that overload (which is LLDB) actually prefers
getting this callback.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D112796