[clangd] Add line and column number to the index symbol.

Summary:
LSP is using Line & column as symbol position, clangd needs to transfer file
offset to Line & column when sending results back to LSP client, which is a high
cost, especially for finding workspace symbol -- we have to read the file
content from disk (if it isn't loaded in memory).

Saving these information in the index will make the clangd life eaiser.

Reviewers: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45513

llvm-svn: 329997