…rict DWARF v2 mode

During testing of llvm#96202 we found that
when clang set to DWARF v2 was used to build the test file, lldb could not tell
that the unsigned enum type was in fact unsigned. So it defaulted to signed and
printed the wrong value.

The reason for this is that DWARFv2 does not include DW_AT_type in DW_TAG_enumeration_type.
This was added in DWARF v3:
"The enumeration type entry may also have a DW_AT_type attribute which refers to the
underlying data type used to implement the enumeration.

In C or C++, the underlying type will be the appropriate integral type determined by the
compiler from the properties of the enumeration literal values."

I noticed that gcc does emit this attribute for DWARF v2 but not when strict
DWARF is requested (more details in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16063#c7).

This patch changes to clang to do the same. This will improve the experience of anyone
using tools that can understand the attribute but for whatever reason are stuck
building binaries containing v2 only.

You can see a current clang/gcc comparison here: https://godbolt.org/z/eG9Kc9WGf

https://reviews.llvm.org/D42734 added the original code that emitted this
for >= v3 only.