[DWARF] Versioning for DWARF constants; verify FORMs

Associate the version-when-defined with definitions of standard DWARF
constants.  Identify the "vendor" for DWARF extensions.
Use this information to verify FORMs in .debug_abbrev are defined as
of the DWARF version specified in the associated unit.
Removed two tests that had specified DWARF v1 (which essentially does
not exist).

Differential Revision: http://reviews.llvm.org/D30785

llvm-svn: 300875