c-family: Avoid unnecessary work when -Wpragmas is being ignored

c-family: Avoid unnecessary work when -Wpragmas is being ignored

This speeds up handle_pragma_diagnostic by avoiding computing a spelling
suggestion for an unrecognized option inside a #pragma directive when
-Wpragmas warnings are being suppressed.

In the range-v3 library, which contains many instances of

  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wpragmas"
  #pragma GCC diagnostic ignored "-Wfoo"
  ...
  #pragma GCC diagnostic pop

(where -Wfoo stands for a warning option we don't recognize), this
reduces compile time by 33% for some of its tests.

gcc/c-family/ChangeLog:

	* c-pragma.c (handle_pragma_diagnostic): Split the
	unknown-option -Wpragmas diagnostic into a warning and a
	subsequent note containing a spelling suggestion.  Avoid
	computing the suggestion if -Wpragmas warnings are being
	suppressed.

gcc/testsuite/ChangeLog:

	* gcc.dg/pragma-diag-6.c: Adjust expected diagnostics
	accordingly.