Open Radar

 
When using clang modules, the macros defined in tgmath.h can’t be used in source code.

Steps to Reproduce:
Given this minimal sample:

```
#import <tgmath.h>
#import <Foundation/Foundation.h>
int main() { float f = round(0.42f); }
```
(Also attached as test.m)

The `round()` function is defined in math.h as returning a double, it’s overridden by the type-generic version in tgmath.h that returns a float. 

When compiling with `clang -Wconversion test.m`, there’s no conversion warning, which is the expected behaviour.

When compiling with `clang -fmodules -Wconversion test.m`, thus enabling modules, we have this warning:
```
test.m:3:24: warning: implicit conversion loses floating-point precision: 'double' to 'float' [-Wconversion]
int main() { float f = round(0.42f); }
                   ~   ^~~~~~~~~~~~
```
as if we didn’t import tgmath.h

Expected Results:
Using modules shouldn’t prevent importing tgmath.h

Actual Results:
tgmath.h macros are not usable.


Version:
Xcode 6.1, Mac OS X 10.10.1

Notes:
My best guess is that the math.h functions are builtin functions in clang, and somehow when using modules the builtin functions can’t be overridden.