Parser cleanup

Ported some changes from Angle. Added 4 new functions to
ParserHelper to remove some code from glslang.y. Added
some extra checks regarding qualifiers and precision.

Change-Id: I2856a764749bef0df500891eb4c003211e634673
Reviewed-on: https://swiftshader-review.googlesource.com/4900
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>