[codeview] Some cleanup of Symbol Records.

* Reworks the CVSymbolTypes.def to work similarly to TypeRecords.def.
* Moves some enums from SymbolRecords.h to CodeView.h to maintain
  consistency with how we do type records.
* Generalize a few simple things like the record prefix
* Define the leaf enum and the kind enum similar to how we do with tyep
  records.

Differential Revision: http://reviews.llvm.org/D20342
Reviewed By: amccarth, rnk

llvm-svn: 269867