Don't crash on undefined function-like macros

[tannergooding]

Using an undefined function-like macro in another macro's body crashed the generator with InvalidCastException: Unable to cast object of type 'ClangSharp.Stmt' to type 'ClangSharp.Expr':

#define ADDRESS_IN_USE TESTRESULT_FROM_WIN32(10048)   // TESTRESULT_FROM_WIN32 is not defined

The generator synthesizes a ClangSharpMacro_ VarDecl and lazily resolves its initializer via VarDecl.InitFactory, which hard-cast the cursor through GetOrCreate<Expr>. When the initializer resolves to a plain Stmt rather than an Expr, that cast threw.

InitFactory now resolves via GetOrCreate<Stmt>(...) as Expr, so a non-Expr initializer yields null instead of throwing. This is behavior-preserving for real initializers, which are always Expr. The macro handling in VisitVarDecl gracefully skips such a macro and emits a warning diagnostic instead of crashing, and the existing RecoveryExpr skip now emits the same warning rather than silently dropping the macro (modern Clang represents these unresolved invocations as a RecoveryExpr).

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Added UndefinedFunctionLikeMacroTest to the VarDeclaration golden suite (Base + all 16 configuration variants), asserting no crash, empty output, and the expected warning diagnostic.

Note: on the pinned libClang 21.1.8 the exact Stmt -> Expr cast is no longer produced (Clang yields a RecoveryExpr/CallExpr), so this hardens the reported cast site defensively and locks in the graceful warn-and-skip behavior via the new regression test.

Fixes #222