Avoid fallthrough in header, to allow external users of Clang libraries to
build with Clang's -Wimplicit-fallthrough warning enabled. The fallthrough
was not making this code better.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183952 91177308-0d34-0410-b5e6-96231b3b80d8