[CIR][Transforms] Fix flattening for TryOp with empty catch region (llvm#1646)

Currently, the following code snippet crashes during flattening, before
lowering to llvm:
```
struct S {
  int a, b;
};

void foo() {
  try {
    S s{1, 2};
  } catch (...) {
  }
}
```
Command to reproduce:
```
clang tmp.cpp -Xclang -fclangir -Xclang -emit-cir-flat -S -o -
```
The crash happens when flattening a TryOp with an empty catch region and
[building
catchers](https://github.com/llvm/clangir/blob/791c327da623e4cb1c193422f4b7a555f572b70a/clang/lib/CIR/Dialect/Transforms/FlattenCFG.cpp#L423).
Something like:
```
"cir.try"() ({
}, {
}) : () -> ()
```
the crash happens at
[`tryOp.isCatchAllOnly()`](https://github.com/llvm/clangir/blob/791c327da623e4cb1c193422f4b7a555f572b70a/clang/lib/CIR/Dialect/Transforms/FlattenCFG.cpp#L441C39-L441C61)
to be specific, because the catch types attribute list is empty.

The fix is simple - adding a check for an empty/non-existent catch
region before building the catch clauses.

This PR adds this fix and one test.

**Side-note:** This enables `push_back` for `std::vector` to be lowered
to llvm, for example:
```
#include <vector>

void foo() {
  std::vector<int> v;
  v.push_back(1);
}
```

Author: bruteforceboy

clang/lib/CIR/Dialect/Transforms/FlattenCFG.cpp

@@ -435,6 +435,10 @@ class CIRTryOpFlattening : public mlir::OpRewritePattern<cir::TryOp> {
     if (auto tryBodyYield = dyn_cast<cir::YieldOp>(afterBody->getTerminator()))
       rewriter.replaceOpWithNewOp<cir::BrOp>(tryBodyYield, afterTry);
 
+    mlir::ArrayAttr catches = tryOp.getCatchTypesAttr();
+    if (!catches || catches.empty())
+      return;
+
     // Start the landing pad by getting the inflight exception information.
     mlir::Block *nextDispatcher =
         buildLandingPads(tryOp, rewriter, beforeCatch, afterTry, callsToRewrite,

clang/test/CIR/CodeGen/try-catch.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fcxx-exceptions -fexceptions -mconstructor-aliases -fclangir -emit-cir %s -o %t.cir
 // RUN: FileCheck --input-file=%t.cir %s
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fcxx-exceptions -fexceptions -mconstructor-aliases -fclangir -emit-cir-flat %s -o %t.cir.flat
+// RUN: FileCheck --check-prefix=FLAT --input-file=%t.cir.flat %s
 
 double division(int a, int b);
 
@@ -168,3 +170,38 @@ void tc7() {
 // CHECK:     cir.return
 // CHECK:   }
 // CHECK: }
+
+struct S2 {
+  int a, b;
+};
+
+void tc8() {
+  try {
+    S2 s{1, 2};
+  } catch (...) {
+  }
+}
+
+// CHECK: cir.scope {
+// CHECK:   %[[V0:.*]] = cir.alloca !rec_S2, !cir.ptr<!rec_S2>, ["s"] {alignment = 4 : i64}
+// CHECK:   cir.try {
+// CHECK:     %[[V1:.*]] = cir.const #cir.const_record<{#cir.int<1> : !s32i, #cir.int<2> : !s32i}> : !rec_S2
+// CHECK:     cir.store align(4) %[[V1]], %[[V0]] : !rec_S2, !cir.ptr<!rec_S2>
+// CHECK:     cir.yield
+// CHECK:   }
+// CHECK: }
+
+// FLAT: cir.func @_Z3tc8v()
+// FLAT:   %[[V0:.*]] = cir.alloca !rec_S2, !cir.ptr<!rec_S2>, ["s"] {alignment = 4 : i64}
+// FLAT:   cir.br ^bb[[#B1:]]
+// FLAT: ^bb[[#B1]]:
+// FLAT:   cir.br ^bb[[#B2:]]
+// FLAT: ^bb[[#B2]]:
+// FLAT:   %[[V1:.*]] = cir.const #cir.const_record<{#cir.int<1> : !s32i, #cir.int<2> : !s32i}> : !rec_S2
+// FLAT:   cir.store align(4) %[[V1]], %[[V0]] : !rec_S2, !cir.ptr<!rec_S2>
+// FLAT:   cir.br ^bb[[#B3:]]
+// FLAT: ^bb[[#B3]]:
+// FLAT:   cir.br ^bb[[#B4:]]
+// FLAT: ^bb[[#B4]]:
+// FLAT:   cir.return
+// FLAT: }