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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373665 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Sema/Overload.h

@@ -826,10 +826,10 @@ class Sema;
 
     unsigned getNumParams() const {
       if (IsSurrogate) {
-        auto STy = Surrogate->getConversionType();
+        QualType STy = Surrogate->getConversionType();
         while (STy->isPointerType() || STy->isReferenceType())
           STy = STy->getPointeeType();
-        return STy->getAs<FunctionProtoType>()->getNumParams();
+        return STy->castAs<FunctionProtoType>()->getNumParams();
       }
       if (Function)
         return Function->getNumParams();