Merge remote-tracking branch 'aosp/upstream-master'

Pulls in the following:
68b8e94 Invoke python via PYTHON_EXE discovered by cmake
8541199 Build SPIRV-Tools file Instruction.cpp instead of Id.cpp
620166a Use with clause to handle files.
b077f3e Avoid redefining builtin dir and break long comment lines.

Change-Id: If9ee4c343a3d5d2225653baa286d116911e9ed65