[WebAssembly] `__has_attribute(musttail)` evaluates to true even when `musttail` is unsupported

[ahatanak]

$ cat test.cpp

#if __has_attribute(musttail)
#warning HAS IT
#else
#warning DOES NOT HAVE
#endif
void F(void) { [[clang::musttail]] return F(); }

$ clang++ -std=c++20  -target wasm32-unknown-unknown -c test.cpp
test.cpp:2:2: warning: HAS IT [-W#warnings]
    2 | #warning HAS IT
      |  ^
test.cpp:6:6: error: WebAssembly 'tail-call' feature not enabled
    6 | void F(void) { [[clang::musttail]] return F(); }
      |      ^
1 warning and 1 error generated.

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Akira Hatanaka (ahatanak)

$ cat test.cpp ``` #if __has_attribute(musttail) #warning HAS IT #else #warning DOES NOT HAVE #endif void F(void) { [[clang::musttail]] return F(); } ```

$ clang++ -std=c++20 -target wasm32-unknown-unknown -c test.cpp
test.cpp:2:2: warning: HAS IT [-W#warnings]
2 | #warning HAS IT
| ^
test.cpp:6:6: error: WebAssembly 'tail-call' feature not enabled
6 | void F(void) { [[clang::musttail]] return F(); }
| ^
1 warning and 1 error generated.

[dschuff]

Yeah, probably this should evaluate to true only when the tail-call feature is enabled.