Bitcode reader: replace DecodeChar6() with a lookup table (NFC)

Summary: Measured to be more performant when reading bitcode.

Reviewers: rafael, dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16285

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 260455