glsl: Properly add functions during lazy built-in prototype importing.

glsl: Properly add functions during lazy built-in prototype importing.

The original lazy built-in importing patch did not add the newly created
function to the symbol table, nor actually emit it into the IR stream.

Adding it to the symbol table is non-trivial since importing occurs when
generating some ir_call in a nested scope.  A new add_global_function
method, backed by new symbol_table code in the previous patch, handles
this.

Fixes bug #32030.