Improve the diagnostic and recovery for missing colons after 'case'

and 'default' statements, including a Fix-It to add the colon:

test/Parser/switch-recovery.cpp:13:12: error: expected ':' after 'case'
    case 17 // expected-error{{expected ':' after 'case'}}
           ^
           :
test/Parser/switch-recovery.cpp:16:12: error: expected ':' after 'default'
    default // expected-error{{expected ':' after 'default'}}
           ^
           :

llvm-svn: 122522