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.
