glsl: Add method to determine whether an expression contains the sequence operator

glsl: Add method to determine whether an expression contains the sequence operator

This will be used in the next patch to enforce some language sematics.

v2: Fix inverted logic in
ast_function_expression::has_sequence_subexpression.  The method
originally had a different name and a different meaning.  I fixed the
logic in ast_to_hir.cpp, but I only changed the names in
ast_function.cpp.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com> [v1]
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>