[Clang][NFC] Change uses of getAs() to castAs() where the target type is assured.

Static analysis identified two uses of getAs() for which the result pointer
was unconditionally dereferenced. Source code inspection confirmed that the
target type is assured by prior checks. This change replaces these uses of
getAs() with castAs().

The first case, in clang/lib/CodeGen/Targets/AArch64.cpp, performs a cast
to BuiltinType following a check for isBuiltinType().

The second case, in clang/lib/Sema/SemaTemplateVariadic.cpp, performs a cast
to PackExpansionType on the result of a call to getAsType() on an object of
type TemplateArgument following confirmation that isPackExpansion() returned
true and that getKind() returned TemplateArgument::Type. Inspection of
isPackExpansion() revealed that it only returns true when the template argument
kind is TemplateArgument::Type if isa<PackExpansionType>(getAsType()) is true.

Author: tahonermann

clang/lib/CodeGen/Targets/AArch64.cpp

@@ -763,7 +763,7 @@ bool AArch64ABIInfo::passAsPureScalableType(
     return false;
 
   bool isPredicate;
-  switch (Ty->getAs<BuiltinType>()->getKind()) {
+  switch (Ty->castAs<BuiltinType>()->getKind()) {
 #define SVE_VECTOR_TYPE(Name, MangledName, Id, SingletonId)                    \
   case BuiltinType::Id:                                                        \
     isPredicate = false;                                                       \

clang/lib/Sema/SemaTemplateVariadic.cpp

@@ -834,7 +834,7 @@ bool Sema::CheckParameterPacksForExpansion(
 
             if (TA.getKind() == TemplateArgument::Type)
               return !TA.getAsType()
-                          ->getAs<PackExpansionType>()
+                          ->castAs<PackExpansionType>()
                           ->getNumExpansions();
 
             if (TA.getKind() == TemplateArgument::Expression)