Improve warning configuration for clang.

Clang 4.0 introduced -Wdouble-promotion, so check for and disable
that on clang as we do on gcc.

The old style casts warning is already disabled globally in this file
for clang, so it doesn't need to be re-disabled within this scope.

imgui_draw.cpp

@@ -42,6 +42,9 @@
 #if __has_warning("-Wreserved-id-macro")
 #pragma clang diagnostic ignored "-Wreserved-id-macro"      // warning : macro name is a reserved identifier                //
 #endif
+#if __has_warning("-Wdouble-promotion")
+#pragma clang diagnostic ignored "-Wdouble-promotion"       // warning: implicit conversion from 'float' to 'double' when passing argument to function
+#endif
 #elif defined(__GNUC__)
 #pragma GCC diagnostic ignored "-Wunused-function"          // warning: 'xxxx' defined but not used
 #pragma GCC diagnostic ignored "-Wdouble-promotion"         // warning: implicit conversion from 'float' to 'double' when passing argument to function
@@ -69,7 +72,6 @@ namespace IMGUI_STB_NAMESPACE
 
 #ifdef __clang__
 #pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wold-style-cast"         // warning : use of old-style cast                              // yes, they are more terse.
 #pragma clang diagnostic ignored "-Wunused-function"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #endif