[Build] Add rule to generate fully-expanded .td file

I found it useful in the past and now again to have a version of the .td file
where all the records are expanded.  This adds a makefile rule to generate
this on demand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208056 91177308-0d34-0410-b5e6-96231b3b80d8