diagnostics: move diagnostic.c to diagnostics/context.cc

diagnostics: move diagnostic.c to diagnostics/context.cc

No functional change intended.

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Replace diagnostic.o with
	diagnostics/context.o.

gcc/c-family/ChangeLog:
	* c-common.cc (c_family_tests): Add include of
	"diagnostics/diagnostics-selftests.h".  Replace
	c_diagnostic_cc_tests with
	diagnostics::selftest::context_cc_tests.
	* c-common.h: Drop c_diagnostic_cc_tests decl.

gcc/ChangeLog:
	* diagnostic.h (num_digits): Move to...
	(diagnostics::num_digits): ...here.
	(get_cwe_url): Move to...
	(diagnostics::get_cwe_url): ...here.
	(maybe_line_and_column): Move to...
	(diagnostics::maybe_line_and_column): ...here.
	* diagnostic.cc: Move to...
	* diagnostics/context.cc: ...here.
	* diagnostics/diagnostics-selftests.cc
	(run_diagnostics_selftests): Add comment about context_cc_tests.
	* diagnostics/diagnostics-selftests.h: Add context_cc_tests decl.
	* diagnostics/sarif-sink.cc: Update for file rename.
	* diagnostics/source-printing.cc: Update for num_digits moving to
	diagnostics::num_digits.
	* errors.cc: Update for file rename.
	* input.cc: Update for num_digits moving to
	diagnostics::num_digits.

libcpp/ChangeLog:
	* charset.cc: Update comment for file rename.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>