…ifiers

The low-level accessor we used previously is commented by LLVM/Clang to
suggest asking ASTContext instead.  Starting with LLVM/Clang 22, this is
mandatory for `TagDecl` types.

LLVM/Clang commit `91cdd35008e9` ([clang] Improve nested name specifier
AST representation, 2025-08-09) renamed `getDecl` to `getOriginalDecl`.
The commit message says it will be reverted before release, so we can
revert here when that happens.

LLVM/Clang 22 changed the AST's representation of elaborated tag types
in commit `91cdd35008e9` ([clang] Improve nested name specifier AST
representation, 2025-08-09).

LLVM/Clang 22 introduced a `PredefinedSugarType` in commit `7c402b8b81d2`
([Clang] Make the SizeType, SignedSizeType and PtrdiffType be named sugar
types, 2025-07-19).

Since LLVM/Clang commit `91cdd35008e9` ([clang] Improve nested name
specifier AST representation, 2025-08-09) and commit `0a675f553caf`
([clang] fix uniquing of some TagTypes created from the injected class
name, 2025-08-26), Clang allocates separate `RecordType` instances for a
record decl and its injected class name.  Replace the latter with the
former, even in indirection types created from them.

CastXML now compiles and passes all tests against this version.