Fix compilation of '#pragma GCC system_header' in google3.

GCC doesn't recognize "-Wpragma-system-header-outside-header" flag
and report an error if we try to use
`#pragma clang diagnostic ignored` to suppress it.

So we use
```
#pragma clang diagnostic ignored "-Wpragma-system-header-outside-header"
```
only when building with clang (we build with clang in google3)

Bug: N/A
Change-Id: I44f166fcf1901336a97b838add0e868d78e1183d