This reverts commit ef82063207.
- It conflicts with the existing llvm::size in STLExtras, which will now
never be called.
- Calling it without llvm:: breaks C++17 compat
Also clang ClangAttrEmitter for -gen-clang-attr-doc-table to be
like all other tablegen: Produce a .inc file with the generated bits
and put the static parts into a regular .cpp file that includes the
.inc file.