[clang] [Clang] Fix warning for non std functions with name `infinity` (PR #123417)

[Erich Keane via cfe-commits]

================
@@ -216,11 +244,18 @@ int compareit(float a, float b) {
 
 // no-inf-no-nan-warning at +2 {{use of infinity is undefined behavior due to the currently enabled floating-point options}}
 // no-inf-warning at +1 {{use of infinity is undefined behavior due to the currently enabled floating-point options}}
-  double y = i * numeric_limits<double>::infinity();
+  double y = i * std::numeric_limits<double>::infinity();
+
+  y = i * numeric_limits<double>::infinity(); // expected-no-diagnostics
 
 // no-inf-no-nan-warning at +2 {{use of infinity is undefined behavior due to the currently enabled floating-point options}}
 // no-inf-warning at +1 {{use of infinity is undefined behavior due to the currently enabled floating-point options}}
-  j = numeric_limits<float>::infinity();
+  j = std::numeric_limits<float>::infinity();
+
+  j = numeric_limits<float>::infinity(); // expected-no-diagnostics
+
+  y = infinity(); // expected-no-diagnostics
+
----------------
erichkeane wrote:

Ah, in that case it is that the same checking code is being called during instantiation, which it almost definitely has to be (to catch the dependent case).

So what you're getting is:

```
std::numeric_limits<T>::infinity(); << Diagnosed ONLY during "Phase 2" of template instantiation.
  std::numeric_limits<double>::infinity(); << Diagnosed BOTH during Phase1 and Phase2.
```

Unfortunately we don't have a GREAT way to suppress warnings in these cases if you don't have access to the actual TreeTransform calls.  Typically we'd try to put the diagnostic somewhere this is ONLY called in Phase1, plus do it again ONLY if transformation happened during Phase 2.

https://github.com/llvm/llvm-project/pull/123417