[clang-tools-extra] [clangd] Add designator hints for parenthesis aggregate initialization (PR #170642)

[Mythreya Kuricheti via cfe-commits]

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@@ -1822,6 +1821,37 @@ TEST(DesignatorHints, NoCrash) {
                         ExpectedHint{".b=", "b"});
 }
 
+TEST(DesignatorHints, BasicParenInit) {
+  assertDesignatorHints(R"cpp(
+    struct S { 
+      int x;
+      int y;
+      int z; 
+    };
+    S s ($x[[1]], $y[[2+2]], $z[[4]]);
+  )cpp",
+                        ExpectedHint{".x=", "x"}, ExpectedHint{".y=", "y"},
+                        ExpectedHint{".z=", "z"});
+}
+
+TEST(DesignatorHints, BasicParenInitDerived) {
+  assertDesignatorHints(R"cpp(
+    struct S1 {
+      int a;
+      int b;
+    };
+
+    struct S2 : S1 { 
+      int c;
+      int d; 
+    };
+    S2 s2 ({$a[[0]], $b[[0]]}, $c[[0]], $d[[0]]);
+  )cpp",
+                        // ExpectedHint{"S1:", "S1"},
----------------
MythreyaK wrote:

@HighCommander4 suggested we add [this](https://github.com/llvm/llvm-project/pull/170642#discussion_r2594632814) in a separate PR so that it's easy to revert if that hint is not well received. Will create a separate PR for it. 

https://github.com/llvm/llvm-project/pull/170642