[Serialization] write expr dependence bits as a single integer

When exprs are written unabbreviated:
  - these were encoded as 5 x vbr6 = 30 bits
  - now they fit exactly into a one-chunk vbr = 6 bits

clangd --check=clangd/AST.cpp reports ~1% reduction in PCH size
(42826720->42474460)

Differential Revision: https://reviews.llvm.org/D124250