__counted_by (and __sized_by) expressions can have arbitrary C syntax
in them, such as:
void foo(int * __counted_by(*len) p, int *len);
When @_SwififyImport tries to generate Swift code for this, the
expression `*len` leads to a syntax error, since it isn't valid Swift.
This patch adds a check to ensure we only attach the Swiftify macro to
__counted_by expressions that are also syntactically valid in Swift.
rdar://150956352
