This refactoring makes it possible for tests to access the IntermNode
tree produced by compilation by calling compileTree(). Removing
ParseContext usage from OutputHLSL has the additional benefit of better
separation between parsing and output.

BUG=angle:916

Change-Id: Ib40954832316328772a5c1dcbbe6b46b238e4e65
Reviewed-on: https://chromium-review.googlesource.com/249723Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>