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Syndicate, test and fix base64 implementation

Authored by serge-sans-paille on Mar 3 2020, 2:10 AM.

Description

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,
h is
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