When Clang is statically linked against other programs they are unable to
find the headers in Clang's resource directory. Typically the resource
directory is found by searching a path relative to argv[0] but this
would only really work for Clang itself due to each binary having a
separate prefix (and not in Nix because of the full resource directory
being split between multiple derivations and assembled in the wrapper).
Because users of Clang as a library typically only need include in order
for parsing to succeed, let's set that as the resource directory. The LLVM
patch to make this work was sent upstream in llvm/llvm-project#145996,
I intend to land it upstream and drop it from this PR.
Peter Collingbourne
8dc663ea18