Syndicate, test and fix base64 implementation

Move Base64 implementation from clangd/SemanticHighlighting to
llvm/Support/Base64, fix its implementation and provide a decent test suite.

Previous implementation code was using + operator instead of | to combine some
results, which is a problem when shifting signed values. (0xFF << 16) is
implicitly converted to a (signed) int, and thus results in 0xffff0000, which is
negative. Combining negative numbers with a + in that context is not what we
want to do.

This fixes https://github.com/llvm/llvm-project/issues/149.

Differential Revision: https://reviews.llvm.org/D75057