[DebugInfoPDB] Teach Variant to support string types.

The IDiaSymbol::getValue() method returns a variant.  Until now,
I had never encountered a string value, so the Variant wrapper
did not support VT_BSTR.  Now we have need to support string
values, so this patch just adds support for one extra type to
Variant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261152 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed