add a char6 abbrev for bbnames and value names. This represents each character

with 6 bits where possible.  This shrinks kc++ from 3324164B to 3183584B.  The
old VST was:

  Block ID #14 (VALUE_SYMTAB):
         Total Size: 1.26713e+07b/1.58391e+06B/395978W
       Average Size: 5403.53b/675.442B/168.86W
          % of file: 47.6484

The new one is:

  Block ID #14 (VALUE_SYMTAB):
         Total Size: 1.15467e+07b/1.44334e+06B/360834W
       Average Size: 4923.96b/615.495B/153.874W
          % of file: 45.3368

This is 11% smaller than the VST in the bytecode format.

llvm-svn: 36771