[clang] [clang][Sema] Don't emit 'declared here' note for builtin functions with no decl in source (PR #93394)

[S. B. Tam via cfe-commits]

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@@ -5,7 +5,6 @@ void similar() { // expected-note {{'similar' declared here}}
   if constexpr (similer<>) {} // expected-error {{use of undeclared identifier 'similer'; did you mean 'similar'?}}
 }
 void a() { if constexpr (__adl_swap<>) {}} // expected-error{{use of undeclared identifier '__adl_swap'; did you mean '__sync_swap'?}} \
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cpplearner wrote:

```suggestion
void a() { if constexpr (__adl_swap<>) {}} // expected-error{{use of undeclared identifier '__adl_swap'; did you mean '__sync_swap'?}}
```

https://github.com/llvm/llvm-project/pull/93394