Revert r297177: Change LLT constructor string into an LLT-based object ...

More module problems. This time it only showed up in the stage 2 compile of
clang-x86_64-linux-selfhost-modules-2 but not the stage 1 compile.

Somehow, this change causes the build to need Attributes.gen before it's been
generated.

llvm-svn: 297188