[Support] add decodeSLEB128()

We already have routines to encode SLEB128 as well as encode/decode ULEB128.
This last function fills out the matrix.  I'll need this for some llvm-objdump
work I am doing.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217830 91177308-0d34-0410-b5e6-96231b3b80d8