Comment lexing: replace manual comparison with StringRef::find_first_of

This gives an about 1.8% improvement on Clang bootstrap with -Wdocumentation


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171262 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed