glsl: Finish out the reduce/reduce error fixes

glsl: Finish out the reduce/reduce error fixes

Track variables, functions, and types during parsing.  Use this
information in the lexer to return the currect "type" for identifiers.

Change the handling of structure constructors.  They will now show up
in the AST as constructors (instead of plain function calls).

Fixes piglit tests constructor-18.vert, constructor-19.vert, and
constructor-20.vert.  Also fixes bugzilla #29926.

NOTE: This is a candidate for the 7.9 and 7.10 branches.