[clang-include-cleaner] make SymbolLocation a real class, move FindHeaders

- replace SymbolLocation std::variant with enum-exposing version similar to
  those in types.cpp. There's no appropriate implementation file, added
  LocateSymbol.cpp in anticipation of locateDecl/locateMacro.
- FindHeaders is not part of the public Analysis interface, so should not
  be implemented/tested there (just code organization)
- rename findIncludeHeaders->findHeaders to avoid confusion with Include concept

Differential Revision: https://reviews.llvm.org/D137825