Add a class APInt to represent arbitrary precision constant integral values.

Add a class APInt to represent arbitrary precision constant integral values.
It is a functional replacement for common case integer type like "unsigned",
"uint64_t", but also allows non-byte-width integer type and large integer
value types such as 3-bits, 15-bits, or more than 64-bits of precision. For
more details, see pr1043.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33913 91177308-0d34-0410-b5e6-96231b3b80d8