[Docs] Fix missing docs strings for CallingConv.h

Replaces
```
//
```
with
```
///
```
for some code lines to make it visible in the auto-generated documentation.

Reviewed By: dblaikie, MaskRay

Differential Revision: https://reviews.llvm.org/D131152

GitOrigin-RevId: 9dbe839627bebc67b718d9fe7d2d66c9c803cf3a
1 file changed