[clangd] Fix a regression of not showing documentation from forward declarations.

Summary:
There is a regression from https://reviews.llvm.org/D68467. Unlike class
forward declarations, function ducomentation is written in the declaration in
headers, the function definition doesn't contain any documentation, cases like:

```
foo.h
// this is foo.
void foo();
foo.cc

void foo() {}
```
we should still show documentation from the foo declaration.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69961

1 files changed, 10 insertions, 3 deletions

diff --git a/clang-tools-extra/clangd/unittests/IndexTests.cpp b/clang-tools-extra/clangd/unittests/IndexTests.cpp
index 9f0265d4190..e868828d786 100644
--- a/clang-tools-extra/clangd/unittests/IndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/IndexTests.cpp
@@ -408,13 +408,20 @@ TEST(MergeIndexTest, Refs) {
 }
 
 TEST(MergeIndexTest, NonDocumentation) {
+  using index::SymbolKind;
   Symbol L, R;
   L.ID = R.ID = SymbolID("x");
   L.Definition.FileURI = "file:/x.h";
   R.Documentation = "Forward declarations because x.h is too big to include";
-
-  Symbol M = mergeSymbol(L, R);
-  EXPECT_EQ(M.Documentation, "");
+  for (auto ClassLikeKind :
+       {SymbolKind::Class, SymbolKind::Struct, SymbolKind::Union}) {
+    L.SymInfo.Kind = ClassLikeKind;
+    EXPECT_EQ(mergeSymbol(L, R).Documentation, "");
+  }
+
+  L.SymInfo.Kind = SymbolKind::Function;
+  R.Documentation = "Documentation from non-class symbols should be included";
+  EXPECT_EQ(mergeSymbol(L, R).Documentation, R.Documentation);
 }
 
 MATCHER_P2(IncludeHeaderWithRef, IncludeHeader, References, "") {