As of recent, LLVM includes the bit-size as a `DW_AT_bit_size` (and as
part of `DW_AT_name`) of `_BitInt`s in DWARF. This allows us to mark
`_BitInt`s as "reconstitutable" when compiling with
`-gsimple-template-names`. However, before doing so we need to make sure
the `DWARFTypePrinter` can reconstruct template parameter values that
have `_BitInt` type. This patch adds support for printing
`DW_TAG_template_value_parameter`s that have `_BitInt` type. Since
`-gsimple-template-names` only omits template parameters that are `<=
64` bit wide, we don't support `_BitInt`s larger than 64 bits.
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