OverloadCandidate::getNumParams - silence static analyzer...

OverloadCandidate::getNumParams - silence static analyzer getAs<FunctionProtoType> null dereference warning. NFCI.

The static analyzer is warning about a potential null dereference, but we should be able to use castAs<FunctionProtoType> directly and if not assert will fire for us.

Also replaces an auto to make the type more obvious.

llvm-svn: 373665