[clangd] Full support for #import insertions

These are still disabled by default, but will work in ObjC code if you
enable the `-import-insertions` flag.

Completion requires ASTSignals to be available; before ASTSignals are
available, we will always use #include. Once they are available, the
behavior varies as follows:

- For source files, use #import if the ObjC language flag is enabled
- For header files:
  - If the ObjC language flag is disabled, use #include
  - If the header file contains any #imports, use #import
  - If the header file references any ObjC decls, use #import
  - Otherwise, use #include

IncludeFixer support is similar, but it does not rely upon ASTSignals,
instead it does the above checks excluding the scan for ObjC symbols.

Differential Revision: https://reviews.llvm.org/D139458