This diff is for discussion how to address problem with large preamble file.
32 bit offsets can be used for PCH/preamble files below 512M. This diff fixes
crashes and asserts on clangd when preamble file exceeds 512M limit. The asserts
usually look like "LLVM ERROR: Invalid abbrev number". On about 700M preamble
files this patch increases file size on about 4%. I tested this diff on
Clang 8, 9 and master.
Tested on clangd with 700M preamble file.