Enable HLSL legalization

Also added known-good mechanism to fetch latest validated spirv-tools.
Also added -Od and -Os to disable optimizer and optimize for size.

Fetching spirv-tools is optional for both glsl and hlsl. Legalization
of hlsl is done by default if spirv-opt is present at cmake time.
Optimization for glsl is currently done through the option -Os.

Legalization testing is currently only done on four existing shaders.
A separate baseLegalResults directory holds those results. All previous
testing is done with the optimizer disabled.

.appveyor.yml

@@ -25,6 +25,7 @@ matrix:
 # scripts that run after cloning repository
 install:
   - git clone https://github.com/google/googletest.git External/googletest
+  - update_glslang_sources.py
 
 build:
   parallel: true  # enable MSBuild parallel builds