Turn on the uncovered umbrella header warning by default

This warning (under -Wincomplete-umbrella) diagnoses cases that are
difficult to understand without the compiler's help, since the symptom
is likely to be that you are missing the contents of headers that are
mistakenly omitted from a module.  This seems like a good thing to have
on by default (with the usual caveat for -Wsystem-headers).

llvm-svn: 203506