[clangd] Implement VByte PostingList compression

This patch implements Variable-length Byte compression of `PostingList`s
to sacrifice some performance for lower memory consumption.

`PostingList` compression and decompression was extensively tested using
fuzzer for multiple hours and runnning significant number of realistic
`FuzzyFindRequests`. AddressSanitizer and UndefinedBehaviorSanitizer
were used to ensure the correct behaviour.

Performance evaluation was conducted with recent LLVM symbol index (292k
symbols) and the collection of user-recorded queries (7751
`FuzzyFindRequest` JSON dumps):

| Metrics | Before| After | Change (%)
| -----  | -----  | -----   | -----
| Memory consumption (posting lists only), MB  |  54.4 | 23.5 | -60%
| Time to process queries, sec | 7.70 | 9.4 | +25%

Reviewers: sammccall, ioeric

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D52300

llvm-svn: 342965