Problem solved by removing:
```
export CC=/usr/bin/clang
export CXX=/usr/bin/clang++
```
from my .mozconfig file. Then the build defaulted to use a newer clang I had under `$HOME/.mozbuild/clang/bin/`
I'll leave this bug open anyway in case clang-6 is supposed to be supported.
Problem solved by removing: bin/clang+ + .mozbuild/ clang/bin/ `
```
export CC=/usr/bin/clang
export CXX=/usr/
```
from my .mozconfig file. Then the build defaulted to use a newer clang I had under `$HOME/
I'll leave this bug open anyway in case clang-6 is supposed to be supported.