Add compute shader compilation support in the glsl compiler

Support is added for compute shader compilation. There is a small
extension to the parser so that 'local_size_x = ', 'local_size_y = '
and 'local_size_z = ' are supported as layout qualifiers.

A few shader compilation tests are added and one which checks the AST
whether the layout qualifiers are properly parsed.

BUG=angleproject:1442

TEST=angle_unittests
TEST=angle_end2end_tests

Change-Id: I67283797d1cf13fa4ac47faa2a6e66d93a2db867
Reviewed-on: https://chromium-review.googlesource.com/362300
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/compiler/translator/Initialize.cpp b/src/compiler/translator/Initialize.cpp
index 87dc8d6..76bed47 100644
--- a/src/compiler/translator/Initialize.cpp
+++ b/src/compiler/translator/Initialize.cpp
@@ -682,7 +682,9 @@
         symbolTable.insert(ESSL3_BUILTINS, new TVariable(NewPoolTString("gl_VertexID"),
                                                          TType(EbtInt, EbpHigh, EvqVertexID, 1)));
         break;
-
+      case GL_COMPUTE_SHADER:
+          // TODO (mradev): add compute shader built-ins
+          break;
       default:
         assert(false && "Language not supported");
     }