glsl: Generalize processing of variable redeclarations.

glsl: Generalize processing of variable redeclarations.

This patch modifies the get_variable_being_redeclared() function so
that it no longer relies on the ast_declaration for the variable being
redeclared.  In future patches, this will allow
get_variable_being_redeclared() to be used for processing
redeclarations of the built-in gl_PerVertex interface block.

v2: Also make get_variable_being_redeclared() static.

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