libcdecl: Fix regression in function parsing.

libcdecl: Fix regression in function parsing.

Merging the reduce_parentheses and simplify_functions step missed a case
where the function simplification could pull a fake function declarator
up to the current tree position.  Continuing the tree traversal will miss
this, leaving the fake parameter in the tree (which is then rejected as
a syntax error later).

The fix is easy enough, just repeat the whole reduction step until no
changes are needed at the current position before continuing traversal.

Add a new test case that targets this behaviour.