DebugInfo: Partial implementation of DWARF type units.

Emit DW_TAG_type_units into the debug_info section using compile unit
headers. This is bogus/unusable by debuggers, but testable and provides
more isolated review.

Subsequent patches will include support for type unit headers and
emission into the debug_types section, as well as comdat grouping the
types based on their hash. Also the CompileUnit type will be renamed
'Unit' and relevant portions pulled out into respective CompileUnit and
TypeUnit types.

llvm-svn: 195166