[libc] disable -Wtype-limits in wctob

GCC warns:
libc/src/__support/wctype_utils.h:33:20: error: comparison of unsigned
expression in ‘< 0’ is always false [-Werror=type-limits]
   33 |   if (c > 127 || c < 0)
      |                  ~~^~~

Looking into the signedness of wint_t, it looks like depending on the platform,
__WINT_TYPE__ is defined to int or unsigned int depending on the platform.

Link: https://lab.llvm.org/buildbot/#/builders/250/builds/14891/steps/6/logs/stdio

Author: nickdesaulniers

libc/src/__support/wctype_utils.h

@@ -29,9 +29,14 @@ LIBC_INLINE cpp::optional<int> wctob(wint_t c) {
   // This needs to be translated to EOF at the callsite. This is to avoid
   // including stdio.h in this file.
   // The standard states that wint_t may either be an alias of wchar_t or
-  // an alias of an integer type, so we need to keep the c < 0 check.
+  // an alias of an integer type where different platforms define this type with
+  // different signedness, so we need to keep the c < 0 check, hence the
+  // pragmas.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wtype-limits"
   if (c > 127 || c < 0)
     return cpp::nullopt;
+#pragma GCC diagnostic pop
   return static_cast<int>(c);
 }