glsl: add ast/parser support for subroutine parsing storage (v3.2)

glsl: add ast/parser support for subroutine parsing storage (v3.2)

This is the guts of the GLSL parser and AST support for
shader subroutines.

The code creates a subroutine type in the parser, and
uses that there to validate the identifiers. The parser
also distinguishes between subroutine types/function prototypes
/uniforms and subroutine defintions for functions.

Then in the AST conversion it recreates the types, and
stores the subroutine definition info or subroutine info
into the ir_function along with a side lookup table in
the parser state. It also converts subroutine calls into
the enhanced ir_call.

v2: move to handling method calls in
function handling not in field selection.
v3: merge Chris's previous parser patches in here, to
make it clearer what's changed in one place.
v3.1: add more documentation, drop unused include
v3.2: drop is_subroutine_def

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Signed-off-by: Dave Airlie <airlied@redhat.com>