[clangd] Expose value of enumerators to Hover API. (not UI yet)

[sammccall]

Summary

This is part of https://github.com/clangd/clangd/issues/180.

[merge_guards_bot]

Build result: pass - 59972 tests passed, 0 failed and 763 were skipped.
Log files: console-log.txt, CMakeCache.txt

[kadircet]

LGTM, thanks!

clang-tools-extra/clangd/unittests/XRefsTests.cpp

984

could you also add a case checking references to enumconstdecl?
i.e.

Color x = GRE^EN;

[sammccall]

clang-tools-extra/clangd/unittests/XRefsTests.cpp

984

Done, this is consistent with the other tests.

I would like to go in the other direction actually, as this is really testing lots of cases of targetDecl. I think we should verify the types of references there, and simplify these cases. But that should probably be done across the board rather than piecemeal.

clang-tools-extra/clangd/XRefs.cpp

if (const Expr *Init = Var->getInit()) {
       Expr::EvalResult Result;
       if (!Init->isValueDependent() && Init->EvaluateAsRValue(Result, Ctx)) {
         HI.Value.emplace();
         llvm::raw_string_ostream ValueOS(*HI.Value);
         Result.Val.printPretty(ValueOS, const_cast<ASTContext &>(Ctx),
                                Init->getType());
       }
     }
+  } else if (const auto *ECD = dyn_cast<EnumConstantDecl>(D)) {
+    // Dependent enums (e.g. nested in template classes) don't have values yet.
+    if (!ECD->getType()->isDependentType())
+      HI.Value = ECD->getInitVal().toString(10);
   }
 
   HI.Definition = printDefinition(D);
   enhanceFromIndex(HI, D, Index);
   return HI;
 }
 
 /// Generate a \p Hover object given the type \p T.

clang-tools-extra/clangd/unittests/XRefsTests.cpp

void foo())cpp";
        [](HoverInfo &HI) {
          HI.Name = "bar";
          HI.Definition = "int bar = Add<1, 2>::result";
          HI.Kind = SymbolKind::Variable;
          HI.Type = "int";
          HI.NamespaceScope = "";
          HI.Value = "3";
        }},
+      {R"cpp(
+        enum Color { RED, GREEN, };
+        Color x = [[GR^EEN]];
+       )cpp",
+       [](HoverInfo &HI) {
+         HI.Name = "GREEN";
+         HI.NamespaceScope = "";
+         HI.LocalScope = "Color::";
+         HI.Definition = "GREEN";
+         HI.Kind = SymbolKind::EnumMember;
+         HI.Type = "enum Color";
+         HI.Value = "1";
+       }},
       // FIXME: We should use the Decl referenced, even if it comes from an
       // implicit instantiation.
       {R"cpp(
         template<int a, int b> struct Add {
           static constexpr int result = a + b;
         };
         int bar = Add<1, 2>::[[resu^lt]];
         )cpp",