[Swiftify] Do not swiftify non-Swift-like counted_by exprs
__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
(cherry picked from commit e5b1f4a2512a18cb00f3f376636428e668cde8f6)
