[C99] Remove the tgmath.h header

Clang provides a number of headers which are required because the
compiler is the only part of the implementation which can provide the
correct macro definitions. Things like <limits.h> or <stdint.h> are
such headers.

<tgmath.h> is not something the compiler needs to provide macros for;
it is the responsibility of the C Standard Library to provide these
interfaces.

Historically, Clang has provided tgmath.h from a time when C11 was
still new and not all C Standard Libraries (such as the one from MSVC)
had correct support. Today, that's no longer the case. So this change
removes <tgmath.h> from the set of headers Clang provides and it is now
on the user to provide a conforming C Standard Library if they would
like to use this functionality.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -122,6 +122,9 @@ C Language Changes
 - Clang now allows an ``inline`` specifier on a typedef declaration of a
   function type in Microsoft compatibility mode. #GH124869
 - Clang now allows ``restrict`` qualifier for array types with pointer elements (#GH92847).
+- Clang no longer provides the ``tgmath.h`` header as part of the compiler-
+  provided header files. This file needs to be provided by the C Standard
+  Library being compiled against.
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/lib/Headers/CMakeLists.txt

@@ -33,7 +33,6 @@ set(core_files
   __stddef_wint_t.h
   stdint.h
   stdnoreturn.h
-  tgmath.h
   unwind.h
   varargs.h
   )

clang/lib/Headers/module.modulemap

@@ -305,11 +305,6 @@ module _Builtin_stdnoreturn [system] {
   export *
 }
 
-module _Builtin_tgmath [system] {
-  header "tgmath.h"
-  export *
-}
-
 module _Builtin_unwind [system] {
   header "unwind.h"
   export *