c++: diagnosing if-stmt with non-constant branches [PR105050]

c++: diagnosing if-stmt with non-constant branches [PR105050]

When an if-stmt is determined to be non-constant because both of its
branches are non-constant, we issue a somewhat generic error which,
since the error also points to the 'if' token, misleadingly suggests
the condition is at fault:

  constexpr-105050.C:8:3: error: expression ‘<statement>’ is not a constant expression
      8 |   if (p != q && *p < 0)
        |   ^~

This patch clarifies the error message to instead read:

  constexpr-105050.C:8:3: error: neither branch of ‘if’ is a constant expression
      8 |   if (p != q && *p < 0)
        |   ^~

	PR c++/105050

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case IF_STMT>:
	Clarify error message when a if-stmt is non-constant because its
	branches are non-constant.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-105050.C: New test.