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.

SPIRV/GlslangToSpv.h

@@ -48,8 +48,11 @@
 namespace glslang {
 
 struct SpvOptions {
-    SpvOptions() : generateDebugInfo(false) { }
+    SpvOptions() : generateDebugInfo(false), disableOptimizer(true),
+        optimizeSize(false) { }
     bool generateDebugInfo;
+    bool disableOptimizer;
+    bool optimizeSize;
 };
 
 void GetSpirvVersion(std::string&);