Syndicate, test and fix base64 implementation
llvm/Support/Base64, fix its implementation and provide a decent test suite.
Previous implementation code was using + operator instead of | to combine
results, which is a problem when shifting signed values. (0xFF << 16) is
implicitly converted to a (signed) int, and thus results in 0xffff0000,
negative. Combining negative numbers with a + in that context is not what we
want to do.
This is a recommit of 5a1958f2673f8c771e406a7e309e160b432c9a79 with UB removved.
This fixes https://github.com/llvm/llvm-project/issues/149.
Differential Revision: https://reviews.llvm.org/D75057