* CPPCompiler -> DownstreamCompiler

* Added DownstreamCompileResult to start abstraction such that we don't need files.

* * Split out slang-blob.cpp
* Made CompileResult hold a DownstreamCompileResult - for access to binary or ISlangSharedLibrary

* Keep temporary files in scope.

* Add a hash to the hex dump stream.

* Move all file tracking into DownstreamCompiler.

* WIP support for nvrtc.

* WIP: Adding support for nvrtc compiler.
Adding enum types, wiring up the nvrtc into slang.

* Fix remaining CPPCompiler references.

* Fix order issue on target string matching.

* Use ISlangSharedLibrary for nvrtc.

* Use DownstreamCompiler for nvrtc.

* WIP first pass at compilation win nvrtc.

* Added testing if file is on file system into CommandLineDownstreamCompiler.
Added sourceContentsPath.

* Make test cuda-compile.cu work by just compiling not comparing output.

* Genearlize DownstreamCompiler usage.

* Fix warning on clang.

* Remove CompilerType from DownstreamCompiler.

* Use DownstreamCompiler interface for all compilers.

NOTE for FXC, DXC and GLSLANG this doesn't mean using 'compile' - it's still extracting functions from shared library.

* Replace DownstreamCompiler::SourceType -> SlangSourceLanguage

* Replace _canCompile with something data driven.

* Fix compiling on gcc/clang for DownstreamCompiler.

* Moved some text conversions into DownstreamCompiler.

* Fix problem on non-vc builds with not having return on locateCompilers for VS.

* Change so no warning for code not reachable on locateCompilers for vs.