glsl/linker: Check the subroutine associated functions names

glsl/linker: Check the subroutine associated functions names

>From Section 6.1.2 (Subroutines) of the GLSL 4.00 specification

    "A program will fail to compile or link if any shader
     or stage contains two or more functions with the same
     name if the name is associated with a subroutine type."

v2:
  - error out earlier (Tapani)
  - style fixes (Iago)

Fixes:
    * no-overloads.vert

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108109
Signed-off-by: Vadym Shovkoplias <vadym.shovkoplias@globallogic.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>