[libclang] Remove comma from the blacklist of characters that prevent a comment to be attached to a decl.

It's common to use an availability function macro at the start of a decl.
rdar://13965065

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187230 91177308-0d34-0410-b5e6-96231b3b80d8