[LifetimeSafety] Run analysis in post-order of CallGraph for better annotation propagation (#174178)

Add functionality to analyze functions in the post-order of the call
graph.

The PR includes the following changes:

1. **Call Graph Generation**: Uses `clang::CallGraph` and
`addToCallGraph` to generate a call graph.
2. **Topological Traversal**: Uses `llvm::post_order` to iterate through
the CallGraph.
3. **New Frontend Flag**: The post-order analysis is enabled via a new
frontend flag `-fexperimental-lifetime-safety-inference-post-order`

Example:

```css
#include <iostream>
#include <string>

  std::string_view f_1(std::string_view a);
  std::string_view f_2(std::string_view a);
  std::string_view f_f(std::string_view a);

  std::string_view f_f(std::string_view a) {
    std::string stack = "something on stack";
    std::string_view res = f_2(stack);
    return res;
  }

 std::string_view f_2(std::string_view a) {
    return f_1(a);
  }

 std::string_view f_1(std::string_view a) {
    return a;
  }
```

Ouput:

```
s.cpp:20:26: warning: parameter in intra-TU function should be marked [[clang::lifetimebound]] [-Wexperimental-lifetime-safety-intra-tu-suggestions]
   20 |     std::string_view f_1(std::string_view a)
      |                          ^~~~~~~~~~~~~~~~~~
      |                                             [[clang::lifetimebound]]
s.cpp:22:12: note: param returned here
   22 |     return a;
      |            ^
s.cpp:15:26: warning: parameter in intra-TU function should be marked [[clang::lifetimebound]] [-Wexperimental-lifetime-safety-intra-tu-suggestions]
   15 |     std::string_view f_2(std::string_view a)
      |                          ^~~~~~~~~~~~~~~~~~
      |                                             [[clang::lifetimebound]]
s.cpp:17:12: note: param returned here
   17 |     return f_1(a);
      |            ^~~~~~
s.cpp:11:32: warning: address of stack memory is returned later [-Wexperimental-lifetime-safety-permissive]
   11 |     std::string_view res = f_2(stack);
      |                                ^~~~~
s.cpp:12:12: note: returned here
   12 |     return res;
```

Fixes https://github.com/llvm/llvm-project/issues/172862

---------

Co-authored-by: Utkarsh Saxena <usx@google.com>