Better encapsulation for TParseContext

Changed 15 public members of TParseContext
so that they are now private and added the
appropriate setters/getters, along with the
required code changes in the parser.

Change-Id: I0a3ea67540d165e9837a3fe8e64fda4843a3cf96
Reviewed-on: https://swiftshader-review.googlesource.com/3543
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>