* Use uninstrumented spirv-as for corpus prep
Using an instrumented version of spirv-as for corpus preparation is very
slow, and can also lead to issues where a bug in spirv-as gets picked up
by instrumentation and causes corpus preparation to fail when we would
rather it succeeded despite the bug.
* Disable msan
Disabling msan for now so that fuzzing using other sanitizers can
proceed.
Now that the CMake files for spirv-tools have been fixed, so that when
LIB_FUZZING_ENGINE is set, it is used as the sole way of specifying
fuzzer-related linker flags, the honggfuzz engine should work. This
change reverts the project to use the default set of fuzzing engines,
which includes honggfuzz.
The LIB_FUZZING_ENGINE environment variable is now passed to the CMake
command of spirv-tools as an option, rather than having the environment
variable be queried in the project's CMake files.
Alastair Donaldson