[CIR][Transforms] Fix flattening for TryOp with empty catch region

[bruteforceboy]

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. Something like:

"cir.try"() ({
}, {
}) : () -> ()

the crash happens at tryOp.isCatchAllOnly() 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);
}