[clangd] Use flags from open files when opening headers they include
Currently our strategy for getting header compile flags is something like:
A) look for flags for the header in compile_commands.json
This basically never works, build systems don't generate this info.
B) try to match to an impl file in compile_commands.json and use its flags
This only (mostly) works if the headers are in the same project.
C) give up and use fallback flags
This kind of works for stdlib in the default configuration, and otherwise doesn't.
Obviously there are big gaps here.
This patch inserts a new attempt between A and B: if the header is
transitively included by any open file (whether same project or not),
then we use its compile command.
This doesn't make any attempt to solve some related problems:
- parsing non-self-contained header files in context (importing PP state)
- using the compile flags of non-opened candidate files found in the index
Differential Revision: https://reviews.llvm.org/D97351