An upcoming patch to LLDB will require the ability to decode base64. This...

An upcoming patch to LLDB will require the ability to decode base64. This patch adds support for decoding base64 and adds tests.

Resubmission of https://reviews.llvm.org/D126254 with where decodeBase64Byte is no longer a lambda but a static function. Some compilers have different errors or warnings with respect to what needs to be captured and what doesn't (see comments in https://reviews.llvm.org/D126254 for details).

Differential Revision: https://reviews.llvm.org/D128560