[clangd] Implement findOccurrences interface in dynamic index.

Summary:
Implement the interface in
  - FileIndex
  - MemIndex
  - MergeIndex

Depends on https://reviews.llvm.org/D50385.

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

Subscribers: mgrang, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51279

llvm-svn: 341242
diff --git a/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp b/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
index 145eb80..284149a 100644
--- a/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
+++ b/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
@@ -76,7 +76,8 @@
   SymbolSlab::Builder Slab;
   for (const auto &Sym : Symbols)
     Slab.insert(Sym);
-  return MemIndex::build(std::move(Slab).build());
+  return MemIndex::build(std::move(Slab).build(),
+                         SymbolOccurrenceSlab::createEmpty());
 }
 
 CodeCompleteResult completions(ClangdServer &Server, StringRef TestCode,