Rename overloads of _mesa_glsl_shader_target_name().

Previously, _mesa_glsl_shader_target_name() had an overload for GLenum
and an overload for the gl_shader_type enum, each of which behaved
differently.  However, since GLenum is a synonym for unsigned int, and
unsigned ints are often used in place of gl_shader_type (e.g. in loop
indices), there was a big risk of calling the wrong overloda by
mistake.  This patch gives the two overloads different names so that
it's always clear which one we mean to call.