Adapt clang rC357037: removal of setVirtualFileSystem

Adapt clang rC357037: removal of setVirtualFileSystem

textDocument/didOpen: index related files when a header is opened

Fix MaskRay#180

index.initialBlacklist: ["."] can inhibit initial indexing (useful for larger code bases).
Opened files are still indexed, though.
This heuristic allows related files (a/foo.c a/b/foo.cc) to be indexed when a header (foo.h) is opened.

textDocument/didOpen: index related files when a header is opened

Fix MaskRay#180

index.initialBlacklist: ["."] can inhibit initial indexing (useful for larger code bases).
Opened files are still indexed, though.
This heuristic allows related files (a/foo.c a/b/foo.cc) to be indexed when a header (foo.h) is opened.

Add Apache LICENSE file (MaskRay#121)

Implement textDocument/documentLink

Fix hierarchical document symbol

1. Fixed a bug on building document symbol tree: As sym2ds was updated in
place, nested funcs/types may be moved into children of another
lsDocumentSymbol before itself got processed.

2. Namespaces only have declarations, in the old implementation it wasn't included in the result, making the result less hierarchical. This
commit fixes this by including the declarations of a symbol if no
definitions found.

Simplify semantic highlighting; improve hover of auto &&

Fix typo error (MaskRay#49)

- The word strengthened is spelled mistakenly.
- Unpaired symbol:')' seems to be missing.