C++: provide macro used-before-defined hint (PR c++/72786)

C++: provide macro used-before-defined hint (PR c++/72786)

This patch uses the name_hint/deferred_diagnostic to provide
a message in the C++ frontend if a macro is used before it is defined
e.g.:

test.c:6:24: error: expected ';' at end of member declaration
   virtual void clone() const OVERRIDE { }
                        ^~~~~
                             ;
test.c:6:30: error: 'OVERRIDE' does not name a type
   virtual void clone() const OVERRIDE { }
                              ^~~~~~~~
test.c:6:30: note: the macro 'OVERRIDE' had not yet been defined
test.c:15:0: note: it was later defined here
 #define OVERRIDE override

It's possible to do it from the C++ frontend as tokenization happens
up-front (and hence the macro already exists when the above is parsed);
I attempted to do it from the C frontend, but because the C frontend only
tokenizes on-demand during parsing, the macro isn't known about until
later.

gcc/cp/ChangeLog:
	PR c++/72786
	* name-lookup.c (class macro_use_before_def): New class.
	(lookup_name_fuzzy): Detect macro that were used before being
	defined, and report them as such.

gcc/ChangeLog:
	PR c++/72786
	* spellcheck.h (best_match::blithely_get_best_candidate): New
	accessor.

gcc/testsuite/ChangeLog:
	PR c++/72786
	* g++.dg/spellcheck-macro-ordering-2.C: New test case.
	* g++.dg/spellcheck-macro-ordering.C: Add dg-message directives
	for macro used-before-defined.

libcpp/ChangeLog:
	PR c++/72786
	* include/cpplib.h (cpp_macro_definition_location): New decl.
	* macro.c (cpp_macro_definition): New function.

From-SVN: r254978