Summary: Projects depending on benchmark library fail to build with clang with the following error
```
./folly/Benchark.h:263:16: error: inline asm not supported yet: don't know how to handle tied indirect register inputs
```
I am not an expert here, just tried to take an approach similar to the MSVC compiler clause
Reviewed By: @andralex, @yfeldblum
Differential Revision:
D2368670