Twines: Support numeric conversion directly (uitostr, etc).

Twines: Support numeric conversion directly (uitostr, etc).
 - Provides static constructors for doing number to string conversions without
   using temporaries.

 - There are several ways to do this, I think given the Twine constraints this
   is the simplest one.

 - One FIXME for fast number -> hex conversion.

 - Added another comment on one last major bit of perf work Twines need, which
   is to make raw_svector_ostream more efficient.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77445 91177308-0d34-0410-b5e6-96231b3b80d8