This applies the same fix as cfdb8ac2e61, but this time to libIDE so
that SourceKit and swift-ide-test will also pass in an appropriate
argv[0]. Re-enable the tests that broke.
rdar://problem/24431137
Enum constants are naturally going to be named after their ObjC name,
not their Swift name. As such, ignore the swift_name attr on the enum
decl when calculating the common prefix. It turns out this is actually
simpler anyway as it also bypasses the swift_private handling that the
code was already trying to work around.
