glsl: Add method glsl_type::can_implicitly_convert_to()

glsl: Add method glsl_type::can_implicitly_convert_to()

This method checks if a source type is identical to or can be implicitly
converted to a target type according to the GLSL 1.20 spec, Section 4.1.10
Implicit Conversions.

The following commits use the method for a bugfix:
    glsl: Fix implicit conversions in non-constructor function calls
    glsl: Fix implicit conversions in array constructors

Note: This is a candidate for the 7.10 and 7.11 branches.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>