Improve -Wswitch-enum diagnostic message.

Changing wording to include the word "explicitly" (as in "enumeration value ...
not /explicitly/ handled by switch"), as suggested by Richard Smith.

Also, now that the diagnostic text differs between -Wswitch and -Wswitch-enum,
I've simplified the test cases a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148781 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed