This cleans up ExpressionParser so that the lexer only consumes one extra
token in case the parser finishes. The parser will also finish with
YYACCEPT once it has parsed a complete expression. This will make the
preprocessor to generate a more informative unexpected token error
instead of a syntax error if there are extra tokens after #if.

This will also enable reusing ExpressionParser for parsing expressions in
line directives. The format for a line directive that specifies both line
and file numbers is as follows:

 #if line-expression file_expression

ExpressionParser will need to be run twice for each line: first to parse
line-expression and then to parse file-expression. For that reason, it is
essential that ExpressionParser for line-expression stops before
consuming more than one token of file-expression.

BUG=angleproject:989
TEST=angle_unittests, dEQP-GLES3.functional.shaders.preprocessor.*

Change-Id: I0bb92f733c18891eeddbc61e7c5bebdf1003559a
Reviewed-on: https://chromium-review.googlesource.com/300962Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>