[lldb]/Tablegen] Use ElementType instead of DefaultValueUnsinged

The fourth field in the property struct is the default unsigned or enum
value for all types, except for Array and Dictionary types. For those,
it is the element type. During the tablegen conversion, this was
incorrectly translated to DefaultValueUnsigned with a value
corresponding to the OptionValue: enum type. So for
OptionValue::eTypeString this became DefaultUnsignedValue<16>. This
patch extends the tablegen backend to understand ElementType to express
this as ElementType<"String">.

Differential revision: https://reviews.llvm.org/D76535