Fix "pointer is null" static analyzer warnings. NFCI.

Use castAs<> instead of getAs<> since the pointers are dereferenced immediately and castAs will perform the null assertion for us.

Author: RKSimon

clang/lib/AST/VTableBuilder.cpp

@@ -270,8 +270,8 @@ static BaseOffset
 ComputeReturnAdjustmentBaseOffset(ASTContext &Context,
                                   const CXXMethodDecl *DerivedMD,
                                   const CXXMethodDecl *BaseMD) {
-  const FunctionType *BaseFT = BaseMD->getType()->getAs<FunctionType>();
-  const FunctionType *DerivedFT = DerivedMD->getType()->getAs<FunctionType>();
+  const auto *BaseFT = BaseMD->getType()->castAs<FunctionType>();
+  const auto *DerivedFT = DerivedMD->getType()->castAs<FunctionType>();
 
   // Canonicalize the return types.
   CanQualType CanDerivedReturnType =