Deducing this feature missing feature test macro

[camio]

According to #59619, there is no work remaining to add this feature, yet it still lacks a feature test macro which is being worked around like this in libcxx.

The release notes for clang 18.1 had this to say:

Because the support for this feature is still experimental, the feature test macro __cpp_explicit_this_parameter was not set in this version. However, its support can be tested with __has_extension(cxx_explicit_this_parameter).

If this feature is complete, the feature test macro should be added. If it is incomplete, there should be an issue to point to describing the remaining work.

[camio]

+cc @cor3ntin

[cor3ntin]

there are actually a couple of bugs blocking the setting of the feature test macro.
see #107451

We aim to set it in clang 20.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: David Sankel (camio)

According to https://github.com//issues/59619, there is no work remaining to add this feature, yet it still lacks a feature test macro which is being worked around like [this](https://github.com/llvm/llvm-project/blob/a2ba438f3e5635e368333213914c7452a6a6a2da/libcxx/include/__config#L1228) in libcxx.

The release notes for clang 18.1 had this to say:

> Because the support for this feature is still experimental, the feature test macro __cpp_explicit_this_parameter was not set in this version. However, its support can be tested with __has_extension(cxx_explicit_this_parameter).

If this feature is complete, the feature test macro should be added. If it is incomplete, there should be an issue to point to describing the remaining work.