[clang] Move CWG2390 test into cwg23xx.cpp (llvm#94206)

This patch refactors an existing test for
[CWG2390](https://cplusplus.github.io/CWG/issues/2390.html) "Is the
argument of `__has_cpp_attribute` macro-expanded?" to use `#error`
instead of emitting a variable in IRGen and checking it via FileCheck.
As a bonus, this makes it possible to move the test into `cwg23xx.cpp`.

Author: Endilll

clang/test/CXX/drs/cwg2390.cpp

@@ -317,7 +317,42 @@ namespace cwg2387 { // cwg2387: 9
 #endif
 }
 
-// cwg2390 is in cwg2390.cpp
+namespace cwg2390 { // cwg2390: 14
+// Test that macro expansion of the builtin argument works.
+#define C clang
+#define F fallthrough
+#define CF clang::fallthrough
+
+#if !__has_cpp_attribute(F)
+#error "doesn't have fallthrough"
+#endif
+
+#if !__has_cpp_attribute(C::F)
+#error "doesn't have clang::fallthrough 1"
+#endif
+
+#if !__has_cpp_attribute(clang::F)
+#error "doesn't have clang::fallthrough 2"
+#endif
+
+#if !__has_cpp_attribute(C::fallthrough)
+#error "doesn't have clang::fallthrough 3"
+#endif
+
+#if !__has_cpp_attribute(CF)
+#error "doesn't have clang::fallthrough 4"
+#endif
+
+#define FUNCLIKE1(x) clang::x
+#if !__has_cpp_attribute(FUNCLIKE1(fallthrough))
+#error "doesn't have clang::fallthrough through func-like macro 1"
+#endif
+
+#define FUNCLIKE2(x) _Clang::x
+#if !__has_cpp_attribute(FUNCLIKE2(fallthrough))
+#error "doesn't have clang::fallthrough through func-like macro 2"
+#endif
+} // namespace cwg2390
 
 namespace cwg2394 { // cwg2394: 15