[Clang][NFC] Change uses of getAs() to castAs() where the target type is assured. (#130188)

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.

0 files changed, 0 insertions, 0 deletions