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

[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