ES31: Implement gl_in in Geometry Shader

This patch intends to implement geometry shader built-in interface
block instance gl_in defined in GL_OES_geometry_shader.

1. Add the definition of gl_in and its interface block gl_PerVertex
   into the symbol table.
2. Support gl_Position as a member of gl_in.
3. Set the array size of gl_in when a valid input primitive type is
   known.
4. Add check that it should be a compile error to index gl_in or
   call length() on gl_in without a valid input primitive declaration.

This patch also adds unit tests to cover all these new features.

BUG=angleproject:1941
TEST=angle_unittests

Change-Id: I8da20c943b29c9ce904834625b396aab6302e1e1
Reviewed-on: https://chromium-review.googlesource.com/605059
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/compiler/translator/OutputGLSLBase.cpp b/src/compiler/translator/OutputGLSLBase.cpp
index b7aacee..29da221 100644
--- a/src/compiler/translator/OutputGLSLBase.cpp
+++ b/src/compiler/translator/OutputGLSLBase.cpp
@@ -622,7 +622,8 @@
                 const TField *field               = interfaceBlock->fields()[index->getIConst(0)];
 
                 TString fieldName = field->name();
-                ASSERT(!mSymbolTable->findBuiltIn(interfaceBlock->name(), mShaderVersion));
+                ASSERT(!mSymbolTable->findBuiltIn(interfaceBlock->name(), mShaderVersion) ||
+                       interfaceBlock->name() == "gl_PerVertex");
                 fieldName = hashName(TName(fieldName));
 
                 out << fieldName;