where possible, encode symtab names with 7 bits per char instead of 8. This

where possible, encode symtab names with 7 bits per char instead of 8.  This
shaves 110K off kc++ to 3514K.  Before:

  Block ID #14 (VALUE_SYMTAB):
      Num Instances: 2345
         Total Size: 1.50425e+07b/1.88031e+06B/470077W
       Average Size: 6414.69b/801.837B/200.459W
          % of file: 51.8057
  Tot/Avg SubBlocks: 0/0
    Tot/Avg Abbrevs: 2345/1
    Tot/Avg Records: 120924/51.5667
      % Abbrev Recs: 85.1791

after:

  Block ID #14 (VALUE_SYMTAB):
      Num Instances: 2345
         Total Size: 1.41229e+07b/1.76536e+06B/441341W
       Average Size: 6022.56b/752.82B/188.205W
          % of file: 50.2295
  Tot/Avg SubBlocks: 0/0
    Tot/Avg Abbrevs: 4690/2
    Tot/Avg Records: 120924/51.5667
      % Abbrev Recs: 85.1791

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36758 91177308-0d34-0410-b5e6-96231b3b80d8