[LifetimeSafety] Cross-TU Vs Intra-TU Annotation Suggestions (#171972)

Differentiate between cross-TU and intra-TU annotation suggestions.

This PR refines the lifetime annotation suggestion feature by
introducing a distinction between cross-TU and intra-TU annotation
suggestions. The primary purpose of this change is to differentiate
between suggestions which can be replaced by inference and other
cross-tu suggestions where inference is not sufficient to replace the
need of explicit annotations.

We have introduced two new warning groups under Lifetime Safety
Suggestions:

- `-Wexperimental-lifetime-safety-cross-tu-suggestions`: These are for
functions whose declarations are in header, definition in source file.
These are higher priority as it affects callers in other translation
units. Inference cannot replace these annotations across TU boundaries
(even in its most powerful form). Annotation suggestion is on the header
declaration.

- `-Wexperimental-lifetime-safety-intra-tu-suggestions`: For suggestions
on functions which are in the same TU. These can be considered
lower-priority suggestions as inference can potentially handle these
within the same TU.

Example:

```cpp
// Header file r.h
#include <iostream>
#include <string>

std::string_view public_func(std::string_view a);

inline std::string_view inline_header(std::string_view a) {
    return a;
}

```

```cpp
// Source (cpp file)
#include <iostream>
#include <string>
#include "r.h"

std::string_view public_func(std::string_view a) {
  return a;
}

namespace {
std::string_view private_func(std::string_view a) {
  return a;
}
}
```

The warnings generated are:

```
./r.h:6:39: warning: parameter in intra-TU function should be marked [[clang::lifetimebound]] [-Wexperimental-lifetime-safety-intra-tu-suggestions]
    6 | inline std::string_view inline_header(std::string_view a) {
      |                                       ^~~~~~~~~~~~~~~~~~
      |                                                          [[clang::lifetimebound]]
./r.h:7:12: note: param returned here
    7 |     return a;
./r.h:4:30: warning: parameter in cross-TU function should be marked [[clang::lifetimebound]] [-Wexperimental-lifetime-safety-cross-tu-suggestions]
    4 | std::string_view public_func(std::string_view a);
      |                              ^~~~~~~~~~~~~~~~~~
      |                                                 [[clang::lifetimebound]]
r.cpp:6:10: note: param returned here
    6 |   return a;
      |          ^
r.cpp:10:33: warning: parameter in intra-TU function should be marked [[clang::lifetimebound]] [-Wexperimental-lifetime-safety-intra-tu-suggestions]
   10 |   std::string_view private_func(std::string_view a) {
      |                                 ^~~~~~~~~~~~~~~~~~
      |                                                    [[clang::lifetimebound]]
r.cpp:11:10: note: param returned here
   11 |   return a;
      |          ^
```