[clang] [clang-tools-extra] [llvm] [CLANG] Add warning when INF or NAN are used in a binary operation or as function argument in fast math mode. (PR #76873)

[Aaron Ballman via cfe-commits]

================
@@ -2835,6 +2835,9 @@ class Preprocessor {
     if (Identifier.getIdentifierInfo()->isRestrictExpansion() &&
         !SourceMgr.isInMainFile(Identifier.getLocation()))
       emitRestrictExpansionWarning(Identifier);
+
+    if (Identifier.getIdentifierInfo()->getName() == "INFINITY")
----------------
AaronBallman wrote:

We will?
```
#ifndef _HUGE_ENUF
    #define _HUGE_ENUF  1e+300  // _HUGE_ENUF*_HUGE_ENUF must overflow
#endif

#define INFINITY   ((float)(_HUGE_ENUF * _HUGE_ENUF))
...
#define NAN        ((float)(INFINITY * 0.0F))
```
the multiplication for `INFINITY` won't form an actual infinity value in that 
case, so why would the multiplication with `0.0` not generate a NAN macro that 
folds to 0.0F?

If use of `INFINITY` is undefined behavior when `-fno-honor-infinities` is 
passed... it makes sense to me that the definition of `NAN` using `INFINITY` is 
UB by the same logic.

https://github.com/llvm/llvm-project/pull/76873
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