Bug#877359: still not reproducible

[Rebecca N. Palmer]

> Do you know how to test that ? besides building beignet?

No, sorry - oclgrind is the other package to ship what appear to be 
clang .pch files, but I haven't specifically checked whether it's affected.

beignet currently isn't the easiest test case, as (as noted above) it 
has other reproducibility issues, requiring llvm-bcanalyzer to 
distinguish this bug (in OPENCL_EXTENSION_TYPES) from the ones that are 
beignet's fault (in INPUT_FILES_BLOCK and ORIGINAL_PCH_DIR).

I had been mostly ignoring those because there was no point fixing them 
if this bug was to be left open, but can try to deal with them today.