[libclang] If the code-completion point is inside the preamble, adjust the position at the beginning of the file after the preamble.
    
Otherwise we will not hit the code-completion point.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220136 91177308-0d34-0410-b5e6-96231b3b80d8