Add ast_function::hir

Add ast_function::hir

ast_function::hir consists of bits pulled out of
ast_function_definition::hir.  In fact, the later uses the former to
do a lot of its processing.  Several class private data fields were
added to ast_function to facilitate communicate between the two.

This causes the following tests to pass:

    glslparsertest/shaders/CorrectModule.frag

This causes the following tests to fail.  These shaders were
previously failing to compile, but they were all failing for the wrong
reasons.

    glslparsertest/shaders/function9.frag
    glslparsertest/shaders/function10.frag