c++: Improve the MVP -Wparentheses diagnostic.

c++: Improve the MVP -Wparentheses diagnostic.

I noticed that declarator->parenthesized is, for this warning, only set
to the opening paren.  But we can easily make it a range and generate
a nicer diagnostic.  Moreover, we can then offer a fix-it hint.

TL;DR: This patch changes

mvp3.C:8:7: warning: unnecessary parentheses in declaration of ‘i’ [-Wparentheses]
    8 |   int (i);
      |       ^

to

mvp3.C:8:7: warning: unnecessary parentheses in declaration of ‘i’ [-Wparentheses]
    8 |   int (i);
      |       ^~~
mvp3.C:8:7: note: remove parentheses
    8 |   int (i);
      |       ^~~
      |       - -

Tested by using -fdiagnostics-generate-patch and verifying that the
generated patch DTRT.

gcc/cp/ChangeLog:

	* decl.c (grokdeclarator): Offer a fix-it hint for the "unnecessary
	parentheses in declaration" warning.
	* parser.c (cp_parser_direct_declarator): When setting
	declarator->parenthesized, use a location range.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/mvp3.C: New test.