ada: gcc-if: build proper String_Pointer for Get_External_Name

ada: gcc-if: build proper String_Pointer for Get_External_Name

The compiler is allowed to assume it can access String bounds, such as
the prefix passed to Get_External_Name, even in circumstances in which
the prefix is not going to be used and has_prefix is false, so, from
the C side, we have to build a proper String_Template for the
String_Pointer.

for  gcc/ada/ChangeLog

	* gcc-interface/decl.cc (is_cplusplus_method): Build proper
	String for Get_External_Name.