C++: -Wwrite-strings: use location of string constant

C++: -Wwrite-strings: use location of string constant

Consider:

extern int callee (const char *one, char *two, const char *three);

int test ()
{
  return callee ("first", "second", "third");
}

for which -Wwrite-strings was emitting:

Wwrite-strings.C: In function 'int test()':
Wwrite-strings.C:10:44: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
10 |   return callee ("first", "second", "third");
   |                                            ^

This patch fixes the warning so that it underlines the pertinent argument
at the callsite:

Wwrite-strings.C: In function 'int test()':
Wwrite-strings.C:10:27: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
10 |   return callee ("first", "second", "third");
   |                           ^~~~~~~~

Ideally we ought to also issue a "note" highlighting the pertinent
parameter within the decl, but that's not readily available, so I'm
saving it for another patch.

gcc/cp/ChangeLog:
	* typeck.c (string_conv_p): Extract location from EXP and use it
	in preference to input_location when issuing warnings.

gcc/testsuite/ChangeLog:
	* g++.dg/conversion/Wwrite-strings.C: New test.

From-SVN: r263635