Intern all RecTy subclass instances to avoid duplicates.

Make all of the RecTy constructors private, and use get() factory
methods instead. Return singleton instances when it makes sense.

ListTy instance pointers are stored in the element RecTy instance.

BitsRecTy instance pointers, one per length, are stored in a static vector.

Also unique DefInit instances. A Record has a unique DefInit which
has a unique RecordRecTy instance.

This saves some 200k-300k RecTy allocations when parsing ARM.td. It
reduces TableGen's heap usage by almost 50%.

llvm-svn: 135399