HLSL: Add support for printf().

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.

gtests/Hlsl.FromFile.cpp

@@ -308,6 +308,7 @@ INSTANTIATE_TEST_SUITE_P(
         {"hlsl.pp.vert", "main"},
         {"hlsl.pp.line.frag", "main"},
         {"hlsl.precise.frag", "main"},
+        {"hlsl.printf.comp", "main"},
         {"hlsl.promote.atomic.frag", "main"},
         {"hlsl.promote.binary.frag", "main"},
         {"hlsl.promote.vec1.frag", "main"},