glsl: Parse the GLSL 1.50 GS layout qualifiers.

glsl: Parse the GLSL 1.50 GS layout qualifiers.

Limited semantic checking (compatibility between declarations, checking
that they're in the right shader target, etc.) is done.

v2: Remove stray debug printfs.

v3 (Paul Berry <stereotype441@gmail.com>): Process input layout
qualifiers at ast_to_hir time rather than at parse time, since certain
error conditions depend on the relative ordering between input layout
qualifiers, declarations, and calls to .length().

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>