[Bug]: clang 15 defines `__GXX_RTTI` but doesn't have `cxxabi.h`

### Describe the issue

In layout.h we have
```
if defined(__GXX_RTTI)
#define ABSL_INTERNAL_HAS_CXA_DEMANGLE
#endif

#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
#include
#endif
```
which then falls over since cxxabi.h doesn't exist.

### Steps to reproduce the problem

Build run clang-tidy version 15 or 16 (or presumably build with the corresponding clang version).

### What version of Abseil are you using?

20230125.2

### What operating system and version are you using?

Mac OS 13

### What compiler and version are you using?

clang-tidy 15/16

### What build system are you using?

cmake 3.26.3

### Additional context

Using the conan package.

The issue is in layout.h where __GXX_RTTI detection includes cxxabi.h, but clang 15+ defines __GXX_RTTI without providing that header. Start by examining layout.h and the surrounding build configuration. Check how other projects detect cxxabi.h availability. Run a build with clang 15/16 to see the error, then adjust the preprocessor checks to test for the header's existence, not just the macro.

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