`-itypes-for-extern` rewrites itype parameters even when they don't need to be changed

[john-h-kastner]

For example:

void test(int *a : itype(_Ptr<int>)) {}

When converted with 3c -itypes-for-extern, the function is rewritten even though it hasn't changed.

This can break idempotence in some situations:

#define foo int
void test(foo *a) {}

converts (3c -itypes-for-extern) to

#define foo int
void test(int *a : itype(_Ptr<foo>)) {}

but a further pass through 3c gives

#define foo int
void test(int *a : itype(_Ptr<int>)) {}

A fix should be fairly straight forward. The condition for rewriting with an itype under -itypes-for-extern just needs to have a srcHasItype check. Alternatively, a fix could come via the change suggested by matt here (update: now filed as #740). If the internal constraint solves to WILD, and the special casing in the rewriter is removed, the main itype rewriting logic will correctly notice the existing itype and not rewrite.