[clang] NFC: Simplify the interface to CodeGenModule::GetOrCreateMultiVersionResolver().

Previously, GetOrCreateMultiVersionResolver() required the caller to provide
a GlobalDecl along with an llvm::type and FunctionDecl. The latter two can be
cheaply obtained from the first, and the llvm::type parameter is not always
used, so requiring the caller to provide them was unnecessary and created the
possibility that callers would pass an inconsistent set. This change simplifies
the interface to only require the GlobalDecl value.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D122956