1. constants can never occur in the symbol table.

2. All function-level constants are now incorporated into the module-level
  constant pool, since the compaction table was removed.  Eliminate extra
  work to check for them.

This speeds up the bcwriter from 24.4s to 13.1s on 447.dealII and .73 -> .56s
on kc++ in a release build.

llvm-svn: 34084