Add back fallthrough annotations removed by 7f3afab

[slackito]

The original commit removed them to avoid the dependency on llvm Support because of LLVM_FALLTHROUGH, but this triggers -Wimplicit-fallthrough warnings.

LLVM requires a C++17 compiler now, so we should be able to use the standard [[fallthrough]] attribute.

[pcc]

So can we remove the LLVM_FALLTHROUGH macro entirely now?

[slackito]

The LLVM Coding Standards doc says

Unless otherwise documented, LLVM subprojects are written using standard C++17 code and avoid unnecessary vendor-specific extensions.

Nevertheless, we restrict ourselves to features which are available in the major toolchains supported as host compilers (see Getting Started with the LLVM System page, section Software).

And that Software section in the Getting Started document says:

LLVM is written using the subset of C++ documented in coding standards. To enforce this language version, we check the most popular host toolchains for specific minimum versions in our build systems:

• Clang 5.0
• Apple Clang 10.0
• GCC 7.4
• Visual Studio 2019 16.8

According to https://en.cppreference.com/w/cpp/compiler_support/17, [[fallthrough]] is supported by GCC 7, Clang 3.9, VS 2017 15.0 and Apple Clang. So I'd say yes we can. But that's way out of scope for this PR.