[libclang] Don't pad the main buffer for the preamble.

Padding does not seem to be useful currently, and it leads to bogus location if an error
points to the end of the file.

rdar://15836513

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203370 91177308-0d34-0410-b5e6-96231b3b80d8