This patch enables 'location' layout qualifier for vertex outputs and
fragment shader inputs when the shader version is 3.1 in ANGLE GLSL
compiler and adds the check on location conflicts for these varyings.

According to GLSL ES 3.1 SPEC (Chapter 4.4.1 and Chapter 4.4.2),
'location' layout qualifier is allowed on both inputs and outputs of
vertex and fragment shaders.

'location' layout qualifier on shader interfaces is only valid on shaders
whose version is 3.1 and above. According to GLSL ES 3.0 SPEC, vertex shader
cannot have output layout qualifiers (Chapter 4.3.8.2) and fragment shader
cannot have input layout qualifiers (Chapter 4.3.8.1).

The 'location' qualifier on varyings is used in the shader interface
matching defined in OpenGL ES 3.1. (OpenGL ES 3.1 SPEC Chapter 7.4.1). This
new link rule will be added to Program.cpp in another patch.

For the OpenGL ES 3.1 extension GL_OES_geometry_shader, according to
GL_OES_shader_io_blocks SPEC (Chapter 4.4.1 and Chapter 4.4.2), 'location'
layout qualifier is both valid on geometry shader inputs and outputs. This
feature will be implemented together with other rules on geometry shader
inputs and outputs.

BUG=angleproject:2144
TEST=angle_unittests

Change-Id: I62d85f7144c177448321c2db36ed7aaeaa1fb205
Reviewed-on: https://chromium-review.googlesource.com/645366
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>