[clang] [Clang] [NFC] Fix unintended `-Wreturn-type` warnings everywhere in the test suite (PR #123464)

[via cfe-commits]

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@@ -300,7 +300,11 @@ int test_extract_template(MyMatrix<int, 2, 2> Mat1) {
 using double4x4 = double __attribute__((matrix_type(4, 4)));
 
 template <class R, class C>
-auto matrix_subscript(double4x4 m, R r, C c) -> decltype(m[r][c]) {}
+auto matrix_subscript(double4x4 m, R r, C c) -> decltype(m[r][c]) {
+  // FIXME: We can't actually do 'return m[r][c]' here currently.
+  static double d;
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cor3ntin wrote:

are we supposed to be able to?

https://github.com/llvm/llvm-project/pull/123464