It is valid for an attribute and a uniform to have the same name
as long as they are declared in different stages.

Prior to this patch, if any shader stage declares a uniform we
disallowed reusing that name for an attribute.

From GLSL ES Spec 3.00.6, section 4.3.5:
    If a uniform variable name is declared in one stage (e.g., a vertex
    shader) but not in another (e.g., a fragment shader), then that name
    is still available in the other stage for a different use.

This change allows Egginc game to work with ANGLE

Bug: angleproject:5497
Tests: angle_end2end_tests -
           *UniformVariableNameReuseAcrossStages*
           *GlobalNamesConflict*
Change-Id: I8f5657d40a213600d6fb48f63e37e4cdbae120b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2608865Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>