[cxx-interop] Generate IR for decls called from members (#35056)

Currently the following code doesn't work when `callConstructor()` is called from Swift:
```cc
inline int increment(int value) {
  return value + 1;
}

struct Incrementor {
  int incrementee;
  Incrementor(int value) : incrementee(increment(value)) {}
}

int callConstructor(int value) {
  return Incrementor(value).incrementee;
}
```

The issue is that we don't generate `IR` for the `increment()` function when it's only called from a constructor or a method.
Swift is aware of the existence of `increment()` and we see it in `IR` as `declare incrementEi`, however, as we don't to emit a definition, we get the following error:
```
Incrementor::Incrementor(int): error: undefined reference to 'increment(int)'
```

This PR fixes this by visiting constructors and methods in `IRGen` and calling `HandleTopLevelDecl()` with all used declarations, which results in emitting definitions for the used declarations.

Co-authored-by: Marcel Hlopko <hlopko@google.com>

test/Interop/Cxx/class/inline-function-codegen/Inputs/method-calls-method-from-nested-struct.h

@@ -0,0 +1,15 @@
+#ifndef TEST_INTEROP_CXX_CLASS_INLINE_FUNCTION_THROUGH_MEMBER_INPUTS_METHOD_CALLS_METHOD_FROM_NESTED_STRUCT_H
+#define TEST_INTEROP_CXX_CLASS_INLINE_FUNCTION_THROUGH_MEMBER_INPUTS_METHOD_CALLS_METHOD_FROM_NESTED_STRUCT_H
+
+struct IncrementUser {
+  struct Incrementor {
+    int increment(int t) { return t + 1; }
+  };
+  int callIncrement(int value) { return Incrementor().increment(value); }
+};
+
+inline int callMethod(int value) {
+  return IncrementUser().callIncrement(value);
+}
+
+#endif // TEST_INTEROP_CXX_CLASS_INLINE_FUNCTION_THROUGH_MEMBER_INPUTS_METHOD_CALLS_METHOD_FROM_NESTED_STRUCT_H