v0.15 build failure with Apple Clang

Attempting to upgrade Homebrew's IWYU returned CI build failures here: https://github.com/Homebrew/homebrew-core/pull/65850



Homebrew builds with the system compiler (clang). This returned a long series of errors that look like

```

In file included from /usr/local/opt/llvm/include/llvm/ADT/SmallVector.h:17:

/usr/local/opt/llvm/include/llvm/Support/AlignOf.h:49:3: error: C++ requires a type specifier for all declarations

  alignas(::llvm::detail::AlignerImpl<T, Ts...>) char buffer[sizeof(

  ^

/usr/local/opt/llvm/include/llvm/Support/AlignOf.h:49:49: error: expected ';' at end of declaration list

  alignas(::llvm::detail::AlignerImpl<T, Ts...>) char buffer[sizeof(

                                                ^

                                                ;

In file included from /tmp/include-what-you-use-20201128-99314-l5cdfv/iwyu_ast_util.cc:12:

In file included from /tmp/include-what-you-use-20201128-99314-l5cdfv/iwyu_ast_util.h:19:

In file included from /tmp/include-what-you-use-20201128-99314-l5cdfv/iwyu_port.h:16:

In file included from /usr/local/opt/llvm/include/llvm/Support/raw_ostream.h:16:

In file included from /usr/local/opt/llvm/include/llvm/ADT/SmallVector.h:17:

/usr/local/opt/llvm/include/llvm/Support/AlignOf.h:49:3: error: C++ requires a type specifier for all declarations

  alignas(::llvm::detail::AlignerImpl<T, Ts...>) char buffer[sizeof(

  ^

/usr/local/opt/llvm/include/llvm/Support/AlignOf.h:49:49: error: expected ';' at end of declaration list

  alignas(::llvm::detail::AlignerImpl<T, Ts...>) char buffer[sizeof(

                                                ^

                                                ;

```



Compiling IWYU with LLVM clang instead is successful.



Build logs:

[iwyu-log.tar.gz](https://github.com/include-what-you-use/include-what-you-use/files/5685157/iwyu-log.tar.gz)

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