Use LLVM_DELETED_FUNCTION rather than '// do not implement' comments.

Also removes some redundant DNI comments on function declarations already
using the macro.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175465 91177308-0d34-0410-b5e6-96231b3b80d8
Add a 'no-builtin' attribute if we do not want to simplify calls.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175471 91177308-0d34-0410-b5e6-96231b3b80d8