[cmake] Add clang-headers as a dependency of libclang and if we have to copy them
for the IDE case, also create a symlink inside the libclang.dylib directory.
[libclang] If libclang logging is enabled, print all compiler diagnostics to stderr instead of capturing them.
When looking for overridden ObjC methods, don't ignore 'hidden' ones.

When using modules we should not ignore overridden methods from
categories that are hidden because the module is not visible.
This will give more consistent results (when imports change) and it's more
correct since the methods are indeed overridden even if they are not "visible"
for lookup purposes.

rdar://13350796