Allow -1 to assign max value to unsigned bitfields.

Allow -1 to assign max value to unsigned bitfields.

Silence the -Wbitfield-constant-conversion warning for when -1 or other
negative values are assigned to unsigned bitfields, provided that the bitfield
is wider than the minimum number of bits needed to encode the negative value.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@277796 91177308-0d34-0410-b5e6-96231b3b80d8