Rearrange bitfield to allow for more space in file_idx.

This is an alternate patch for the bug reported in
https://bugs.llvm.org/show_bug.cgi?id=39816 ("lldb doesn't show a file
of line entry for big project"). This limits the number of lines in a
source file to 32M, which I think is reasonable even for preprocessed
source inputs.

Differential Revision: https://reviews.llvm.org/D56218

llvm-svn: 350274