[clangd] Encode Line/Column as a 32-bits integer.

Summary:
This would buy us more memory. Using a 32-bits integer is enough for
most human-readable source code (up to 4M lines and 4K columns).

Previsouly, we used 8 bytes for a position, now 4 bytes, it would save
us 8 bytes for each Ref and each Symbol instance.

For LLVM-project binary index file, we save ~13% memory.

| Before | After |
| 412MB  | 355MB |

Reviewers: sammccall

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53363

llvm-svn: 344735