Add basic support for ShaderDebugInfo and line markers

[nyorain]

I use SPIRV-Cross for shader debugging in vil and have a couple of changes that might find a place upstream. This change

• allows multiple source files per parsed module
• adds the NonSemanticShaderDebugInfo100.h header
• stores the debug source strings attached per source (via OpSource or debug ext instruction)
• stores the line markers per source (via OpLine or debug ext instruction)
• stores debug local variables from the shader-debug-info ext
• stores per SPIRVariable which debug local variables are associated with it
• exposes useful functions from the Compiler base class (get_ir and constant evaluation)

I also added simple tests for the debug-info and line marker functionality.

[CLAassistant]

All committers have signed the CLA.

[HansKristian-Work]

LGTM. Thanks for taking the time to write a test bench.