Introduce PackedVector, useful for storing a vector of values using a specific number...

Introduce PackedVector, useful for storing a vector of values using a specific number of bits for each
value. Both signed and unsigned types can be used, e.g

   PackedVector<signed, 2> vec;

will create a vector accepting values -2, -1, 0, 1. Any other value will hit an assertion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132325 91177308-0d34-0410-b5e6-96231b3b80d8