glsl: Track UBO block names in the symbol table.

glsl: Track UBO block names in the symbol table.

The GLSL 1.40 spec says:

    "Uniform block names and variable names declared within uniform
    blocks are scoped at the program level."

Track the block name in the symbol table and emit errors when conflicts
exist.

Fixes es3conform's uniform_buffer_object_block_name_conflict test, and
fixes the piglit block-name-clashes-with-{variable,function,struct}.vert
tests.

NOTE: This is a candidate for the 9.0 branch.

v2: Fix bad constructor initialization.  Noticed by Topi Pohjolainen.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>