[clang] [WebAssembly] Enable musttail only when tail-call is enabled (PR #163618)

[Aaron Ballman via cfe-commits]

================
@@ -508,6 +509,10 @@ def TargetMicrosoftRecordLayout : TargetArch<["x86", "x86_64", "arm", "thumb",
   let CustomCode = [{ Target.hasMicrosoftRecordLayout() }];
 }
 
+def TargetMustTailAvaiable: TargetArch<!listconcat(TargetARM.Arches, TargetAArch64.Arches, TargetAnyX86.Arches, TargetWebAssembly.Arches, TargetPowerPC.Arches)> {
+  let CustomCode = [{ Target.hasMustTail() }];
+}
+
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AaronBallman wrote:

This is close to what I was thinking, but I was going for something more like this:
```
def TargetMustTailAvaiable:  TargetSpec {
  let CustomCode = [{ Target.hasMustTail() }];
}
```
(so no need to list the architectures)

https://github.com/llvm/llvm-project/pull/163618