Avoid fragile type lookups in GDB pretty printer

Instead of using the type llvm::StringMapEntry<{stringified_value_type}>
use only the base class llvm::StringMapEntryBase and calculate the
offsets of the member variables manually. The approach with stringifying
the name of the value type is pretty fragile as it can easily break with
local and dependent types.

Differential Revision: https://reviews.llvm.org/D94431