Roll back use of #warning for header deprecations

e5ccd668 and
5029dce4 added deprecation warnings to
the <stdbool.h> and <stdnoreturn.h> headers, respectively, because the
headers are deprecated in C2x.

However, there are system headers that include these headers
unconditionally, and #warning diagnostics within system headers are
shown to users instead of suppressed, which means these deprecation
warnings are being triggered in circumstances that users have no
control over except to disable all the warnings through the
_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS macro or other means.

This removes the problematic #warning uses until we find a more
palatable solution.