add an abbreviation for common PARM_VAR_DECL. All but 9 of the

add an abbreviation for common PARM_VAR_DECL.  All but 9 of the
parm var decls in leopard cocoa.h end up using this abbreviation,
which shrinks the bitcode file by about 50K: 7217736->7167120.

Before:
  Block ID #12 (DECLS_BLOCK):
      Num Instances: 1
         Total Size: 2.23595e+07b/2.79494e+06B/698736W
          % of file: 38.7233
      Num SubBlocks: 0
        Num Abbrevs: 0
        Num Records: 139387
      % Abbrev Recs: 0

After:
  Block ID #12 (DECLS_BLOCK):
      Num Instances: 1
         Total Size: 2.02405e+07b/2.53006e+06B/632516W
          % of file: 35.301
      Num SubBlocks: 0
        Num Abbrevs: 1
        Num Records: 139387
      % Abbrev Recs: 19.2902

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70199 91177308-0d34-0410-b5e6-96231b3b80d8