[clang-tidy] modernize-deprecated-headers check should respect extern "C" blocks

The check should not report includes wrapped by `extern "C" { ... }` blocks,
such as:

```lang=C++
#ifdef __cplusplus
extern "C" {
#endif

#include "assert.h"

#ifdef __cplusplus
}
#endif
```

This pattern comes up sometimes in header files designed to be consumed
by both C and C++ source files.
The check now reports false reports when the header file is consumed by
a C++ translation unit.

In this change, I'm not emitting the reports immediately from the
`PPCallback`, rather aggregating them for further processing.
After all preprocessing is done, the matcher will be called on the
`TranslationUnitDecl`, ensuring that the check callback is called only
once.

Within that callback, I'm recursively visiting each decls, looking for
`LinkageSpecDecls` which represent the `extern "C"` specifier.
After this, I'm dropping all the reports coming from inside of it.
After the visitation is done, I'm emitting the reports I'm left with.

For performance reasons, I'm sorting the `IncludeMarkers` by their
corresponding locations.
This makes the scan `O(log(N)` when looking up the `IncludeMarkers`
affected by the given `extern "C"` block. For this, I'm using
`lower_bound()` and `upper_bound()`.

Reviewed By: whisperity

Differential Revision: https://reviews.llvm.org/D125209