[HLSL][SPIRV] Map the arguments in the DXC `spv-extension` option to the SPIR-V backend's `-spirv-ext` options.

[s-perron]

This should be a straight forward mapping. We will need to do the following:

• If there is an a value -fspv-extension=<v0>, then it should be translated to -spirv-ext=+<v0>.
• If the -fspv-extension option is not specificed, then -spirv-ext=all should be passed along.
• Test for appropriate error messages when an unknown extension is used.