[NFC][CLANG] Fix Static Code Analyzer Concerns with dereference null …

…return value in applyObjCTypeArgs()

This patch uses castAs instead of getAs to resolve dereference issue with nullptr boundObjC when calling
canAssignObjCInterfaces() or isObjCIdType() in applyObjCTypeArgs() since getAs returns nullptr.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151964

clang/lib/Sema/SemaType.cpp

@@ -950,7 +950,7 @@ static QualType applyObjCTypeArgs(Sema &S, SourceLocation loc, QualType type,
 
       // Retrieve the bound.
       QualType bound = typeParam->getUnderlyingType();
-      const auto *boundObjC = bound->getAs<ObjCObjectPointerType>();
+      const auto *boundObjC = bound->castAs<ObjCObjectPointerType>();
 
       // Determine whether the type argument is substitutable for the bound.
       if (typeArgObjC->isObjCIdType()) {