[clangd] Perform merge for main file symbols.

[hokein]

Summary

Previously, we randomly pick one main file symbol in dynamic index, we
may loose the ideal symbol (with definition location) in the index.

It fixes the issue where sometimes we fail to go to the symbol definition, see:

1. call go-to-decl on Foo in Foo.cpp
2. jump to Foo.h, call go-to-def on Foo in Foo.h

we can't go back to Foo.cpp -- because we open Foo.cpp, Foo.h in clangd, both
files have Foo symbol (one with def&decl, one with decl only), we randomely
choose one.

[kadircet]

We also make use of PickOne in updatePreamble shouldn't that also cause similar troubles?

[hokein]

In D63425#1546010, @kadircet wrote:

We also make use of PickOne in updatePreamble shouldn't that also cause similar troubles?

Yeah, we have similar problems in updatePreamble, but this is a correctness/performance tradeoff -- we have a large number of symbols in preamble, the cost of merging is too high.

However the number of main file symbols is relatively small (e.g. Sema.cpp only has 99 symbols), the cost is not too high.

clang-tools-extra/clangd/index/FileIndex.cpp

 
 void FileIndex::updateMain(PathRef Path, ParsedAST &AST) {
   auto Contents = indexMainDecls(AST);
   MainFileSymbols.update(
       Path, llvm::make_unique<SymbolSlab>(std::move(Contents.first)),
       llvm::make_unique<RefSlab>(std::move(Contents.second)),
       /*CountReferences=*/true);
   MainFileIndex.reset(
-      MainFileSymbols.buildIndex(IndexType::Light, DuplicateHandling::PickOne));
+      MainFileSymbols.buildIndex(IndexType::Light, DuplicateHandling::Merge));
 }
 
 } // namespace clangd
 } // namespace clang

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

 MATCHER_P(DeclURI, U, "") {
   return llvm::StringRef(arg.CanonicalDeclaration.FileURI) == U;
 }
 MATCHER_P(DefURI, U, "") {
   return llvm::StringRef(arg.Definition.FileURI) == U;
 }
 MATCHER_P(QName, N, "") { return (arg.Scope + arg.Name).str() == N; }
 MATCHER_P(NumReferences, N, "") { return arg.References == N; }
+MATCHER_P(hasOrign, O, "") { return bool(arg.Origin & O); }
 
 namespace clang {
 namespace clangd {
 namespace {
 ::testing::Matcher<const RefSlab &>
 RefsAre(std::vector<::testing::Matcher<Ref>> Matchers) {
   return ElementsAre(::testing::Pair(_, UnorderedElementsAreArray(Matchers)));
 }
TEST(FileIndexTest, ReferencesInMainFileWithPreamble) {
   Index.updateMain(testPath(TU.Filename), AST);
 
   // Expect to see references in main file, references in headers are excluded
   // because we only index main AST.
   EXPECT_THAT(getRefs(Index, findSymbol(TU.headerSymbols(), "Foo").ID),
               RefsAre({RefRange(Main.range())}));
 }
 
+TEST(FileIndexTest, MergeMainFileSymbols) {
+  const char* CommonHeader = "void foo();";
+  TestTU Header = TestTU::withCode(CommonHeader);
+  TestTU Cpp = TestTU::withCode("void foo() {}");
+  Cpp.Filename = "foo.cpp";
+  Cpp.HeaderFilename = "foo.h";
+  Cpp.HeaderCode = CommonHeader;
+
+  FileIndex Index;
+  auto HeaderAST = Header.build();
+  auto CppAST = Cpp.build();
+  Index.updateMain(testPath("foo.h"), HeaderAST);
+  Index.updateMain(testPath("foo.cpp"), CppAST);
+
+  auto Symbols = runFuzzyFind(Index, "");
+  // Check foo is merged, foo in Cpp wins (as we see the definition there).
+  EXPECT_THAT(Symbols, ElementsAre(AllOf(DeclURI("unittest:///foo.h"),
+                                         DefURI("unittest:///foo.cpp"),
+                                         hasOrign(SymbolOrigin::Merge))));
+}
+
 TEST(FileSymbolsTest, CountReferencesNoRefSlabs) {
   FileSymbols FS;
   FS.update("f1", numSlab(1, 3), nullptr, true);
   FS.update("f2", numSlab(1, 3), nullptr, false);
   EXPECT_THAT(
       runFuzzyFind(*FS.buildIndex(IndexType::Light, DuplicateHandling::Merge),
                    ""),
       UnorderedElementsAre(AllOf(QName("1"), NumReferences(0u)),