glsl: Emit function signatures at toplevel, even for built-ins.

glsl: Emit function signatures at toplevel, even for built-ins.

The ast-to-hir conversion needs to emit function signatures in two
circumstances: when a function declaration (or definition) is
encountered, and when a built-in function is encountered.

To avoid emitting a function signature in an illegal place (such as
inside a function), emit_function() checked whether we were inside a
function definition, and if so, emitted the signature before the
function definition.

However, this didn't cover the case of emitting function signatures
for built-in functions when those built-in functions are called from
inside the constant integer expression that specifies the length of a
global array.  This failed because when processing an array length, we
are emitting IR into a dummy exec_list (see process_array_type() in
ast_to_hir.cpp).  process_array_type() later checks (via an assertion)
that no instructions were emitted to the dummy exec_list, based on the
reasonable assumption that we shouldn't need to emit instructions to
calculate the value of a constant.

This patch changes emit_function() so that it emits function
signatures at toplevel in all cases.

This partially fixes bug 38625
(https://bugs.freedesktop.org/show_bug.cgi?id=38625).  The remainder
of the fix is in the patch that follows.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>