Generate load of interpolant for first operand to GLSLstd450
InterpolateAt* SPIR-V ops. This allows the interpolants to
propagate from the input struct in the wrapper around main
into the shader during HLSL legalization. A new pass has been
added to legalization which will remove the load and replace
with the pointer of the load to create valid external
interpolate op.
Fixes#2584
Translate printf() to what GL_EXT_debug_printf has done. HLSL could
define non-constant string variable and we don't have such features
in SPIR-V, so just support constant string variable.
Now that the HLSL source files are part of the `glslang` target (#2271), it makes sense for these to sit in the `glslang` directory.
Changed the case of the directory from `hlsl` to `HLSL` to better match the sibling directories.
