[clangd] Make WorkspaceSymbols request work with empty queries

[kadircet]

Summary

Clangd uses codecompletion limit as the limit for workspacesymbols, so
in theory this should only be an order of magnitude slower than a
codecompletion request with empty identifier (as code completion limits
the available symbols).

This is also what LSP suggests "Clients may send an empty string here to request all symbols.".
Clangd doesn't really fulfill the "all" part of that statement, but we
never do unless user set the index query limit to zero explicitly.

Fixes https://github.com/clangd/clangd/issues/707.

[sammccall]

I've asked on the bug for some more context for the request.
I don't see a problem with this change per se but I'm not sure it'll actually fix the bug.

[sammccall]

LGTM

OK, I think we've established that the request was for a way to dump out clangd's whole index, and workspace/symbol isn't actually a good way to do that.
Nevertheless, this seems useful and endorsed by the spec.

https://reviews.llvm.org/D44882#inline-394499 was the original motivation for the special case, but it predates the spec text and things have probably changed.

clang-tools-extra/clangd/FindSymbols.cpp

llvm::Expected<Location> symbolToLocation(const Symbol &Sym,
   return indexToLSPLocation(
       Sym.Definition ? Sym.Definition : Sym.CanonicalDeclaration, TUPath);
 }
 
 llvm::Expected<std::vector<SymbolInformation>>
 getWorkspaceSymbols(llvm::StringRef Query, int Limit,
                     const SymbolIndex *const Index, llvm::StringRef HintPath) {
   std::vector<SymbolInformation> Result;
-  if (Query.empty() || !Index)
+  if (!Index)
     return Result;
 
   // Lookup for qualified names are performed as:
   // - Exact namespaces are boosted by the index.
   // - Approximate matches are (sub-scope match) included via AnyScope logic.
   // - Non-matching namespaces (no sub-scope match) are post-filtered.
   auto Names = splitQualifiedName(Query);
 

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

TU.Code = R"cpp(
       #include "foo.h"
       )cpp";
   EXPECT_THAT(getSymbols(TU, "::"),
               UnorderedElementsAre(
                   AllOf(QName("Foo"), WithKind(SymbolKind::Class)),
                   AllOf(QName("foo"), WithKind(SymbolKind::Function)),
                   AllOf(QName("ns"), WithKind(SymbolKind::Namespace))));
   EXPECT_THAT(getSymbols(TU, ":"), IsEmpty());
-  EXPECT_THAT(getSymbols(TU, ""), IsEmpty());
+  EXPECT_THAT(getSymbols(TU, ""),
+              UnorderedElementsAre(QName("foo"), QName("Foo"), QName("Foo::a"),
+                                   QName("ns"), QName("ns::foo2")));
 }
 
 TEST(WorkspaceSymbols, Enums) {
   TestTU TU;
   TU.AdditionalFiles["foo.h"] = R"cpp(
     enum {
       Red
     };