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.

Test/baseLegalResults/hlsl.flattenOpaqueInitMix.vert.out

@@ -0,0 +1,43 @@
+hlsl.flattenOpaqueInitMix.vert
+// Module Version 10000
+// Generated by (magic number): 80001
+// Id's are bound by 60
+
+                              Capability Shader
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint Vertex 4  "main" 44
+                              Source HLSL 500
+                              Name 4  "main"
+                              Name 36  "g_tInputTexture_sampler"
+                              Name 37  "g_tInputTexture"
+                              Name 44  "@entryPointOutput"
+                              Decorate 36(g_tInputTexture_sampler) DescriptorSet 0
+                              Decorate 37(g_tInputTexture) DescriptorSet 0
+                              Decorate 44(@entryPointOutput) Location 0
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeSampler
+               7:             TypePointer UniformConstant 6
+               8:             TypeFloat 32
+               9:             TypeImage 8(float) 2D sampled format:Unknown
+              10:             TypePointer UniformConstant 9
+              12:             TypeVector 8(float) 4
+              24:             TypeSampledImage 9
+              28:             TypeVector 8(float) 2
+              30:    8(float) Constant 0
+              35:    8(float) Constant 1056964608
+36(g_tInputTexture_sampler):      7(ptr) Variable UniformConstant
+37(g_tInputTexture):     10(ptr) Variable UniformConstant
+              43:             TypePointer Output 12(fvec4)
+44(@entryPointOutput):     43(ptr) Variable Output
+         4(main):           2 Function None 3
+               5:             Label
+              53:           9 Load 37(g_tInputTexture)
+              54:           6 Load 36(g_tInputTexture_sampler)
+              55:          24 SampledImage 53 54
+              58:   28(fvec2) CompositeConstruct 35 35
+              59:   12(fvec4) ImageSampleExplicitLod 55 58 Lod 30
+                              Store 44(@entryPointOutput) 59
+                              Return
+                              FunctionEnd