Fix -Wswitch to warn about out of bounds enum cases even when there's a default

Fix -Wswitch to warn about out of bounds enum cases even when there's a default

For consistency with GCC & reasonable sanity. The FIXME suggests that the
original author was perhaps using the default check for some other purpose,
not realizing the more obvious limitation/false-negatives it creates, but this
doesn't seem to produce any regressions & fixes the included test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148649 91177308-0d34-0410-b5e6-96231b3b80d8