[clang] Add test for C++ DR2390

[tbaeder]

Summary

DR2390 clarifies that the argument to __has_cpp_attribute() needs to be
macro-expanded. Clang already supports this and tests it explicitly in
clang/test/Preprocessor/has_attribute.cpp.

Copy the test over to clang/test/CXX/drs/ so the make_cxx_drs script
picks it up.

[aaron.ballman]

LGTM, thank you for this!

clang/test/CXX/drs/dr2390.cpp

+// RUN: %clang_cc1 -E -P %s -o - | FileCheck %s
+
+// dr2390: yes
+
+namespace PR48462 {
+// Test that macro expansion of the builtin argument works.
+#define C clang
+#define F fallthrough
+#define CF clang::fallthrough
+
+#if __has_cpp_attribute(F)
+int has_fallthrough;
+#endif
+// CHECK: int has_fallthrough;
+
+#if __has_cpp_attribute(C::F)
+int has_clang_fallthrough_1;
+#endif
+// CHECK: int has_clang_fallthrough_1;
+
+#if __has_cpp_attribute(clang::F)
+int has_clang_fallthrough_2;
+#endif
+// CHECK: int has_clang_fallthrough_2;
+
+#if __has_cpp_attribute(C::fallthrough)
+int has_clang_fallthrough_3;
+#endif
+// CHECK: int has_clang_fallthrough_3;
+
+#if __has_cpp_attribute(CF)
+int has_clang_fallthrough_4;
+#endif
+// CHECK: int has_clang_fallthrough_4;
+
+#define FUNCLIKE1(x) clang::x
+#if __has_cpp_attribute(FUNCLIKE1(fallthrough))
+int funclike_1;
+#endif
+// CHECK: int funclike_1;
+
+#define FUNCLIKE2(x) _Clang::x
+#if __has_cpp_attribute(FUNCLIKE2(fallthrough))
+int funclike_2;
+#endif
+// CHECK: int funclike_2;
+} // namespace PR48462