Fix PR36914 - num_get::get(unsigned) incorrectly handles negative numbers.

This patch corrects num_get for unsigned types to support strings
with a leading `-` character. According to the standard the
number should be parsed as an unsigned integer and then
negated.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@328751 91177308-0d34-0410-b5e6-96231b3b80d8