language-server: Support simple semantic queries. (#7639)

Add support for "jump to declaration", "find references", type
information on hover. This support is strictly single-file for now; only
references and declarations within the same file are found. We could go
a bit beyond that, but to properly handle cross-file references we'll
need to build an index and a compilation database, which is beyond the
scope of this change.

On hover, we provide the type information for the instruction under the
cursor as-is. This is frequently not very useful, as the type of a
function F is simply "<type of F>", but is a starting point for richer
information.

Assisted-by: Claude Code

toolchain/language_server/handle_initialize.cpp

@@ -30,9 +30,14 @@ auto HandleInitialize(
   auto encoding = NegotiatePositionEncoding(params.capabilities);
   context.SetPositionEncoding(encoding);
 
-  llvm::json::Object capabilities{{"documentSymbolProvider", true},
+  llvm::json::Object capabilities{{"declarationProvider", true},
+                                  {"definitionProvider", true},
+                                  {"documentSymbolProvider", true},
+                                  {"hoverProvider", true},
+                                  {"positionEncoding", encoding},
+                                  {"referencesProvider", true},
                                   {"textDocumentSync", /*Incremental=*/2},
-                                  {"positionEncoding", encoding}};
+                                  {"typeDefinitionProvider", true}};
   llvm::json::Object reply{{"capabilities", std::move(capabilities)}};
   on_done(reply);
 }