[clang][HeaderSearch] Treat framework headers as Angled for suggestPath

- Rename the IsSystem flag to be IsAngled since that's how callers
  actually use the flag.

- Since frameworks by convention use <> style includes, make sure
  we treat them as Angled

Also update clangd's custom logic for frameworks accordingly.

Differential Revision: https://reviews.llvm.org/D156704