Xcode: Quote strings containing a tilde (#15672)

Since commit v3.3.0-rc1~183^2 (Xcode: Refine quoting rules for Strings,
2015-04-09) we no longer quote strings containing a period ('.').
However, file names like "icon29x29~ipad.png" still need quoting because
they contain a tilde ('~').  Add tilde to our explicit list of
characters that need quoting because such file names will no longer
happen to be quoted because they contain a period.

Source/cmXCodeObject.cxx

@@ -243,7 +243,7 @@ void cmXCodeObject::PrintString(std::ostream& os,std::string String)
   bool needQuote =
     (String.empty() ||
      String.find("//") != String.npos ||
-     String.find_first_of(" <>+-*=@[](){},") != String.npos);
+     String.find_first_of(" <>+-*=@[](){},~") != String.npos);
   const char* quote = needQuote? "\"" : "";
 
   // Print the string, quoted and escaped as necessary.