glsl: Don't return bad values from _mesa_shader_type_to_index.

This will avoid compiler warnings in the patch that follows.  There
should be no user-visible effect because the change only affects the
behaviour when an invalid enum is passed to
_mesa_shader_type_to_index(), and that can only happen if there is a
bug elsewhere in Mesa.
main: Move MESA_SHADER_TYPES outside of gl_shader_type enum.

This will avoid spurious compiler warnings in the patch that follows.
glsl: Replace _mesa_glsl_parser_targets enum with gl_shader_type.

These enums were redundant.