[DWARF] Fix undefined behaviour in dwarf type printer

The value to be formatted here, Val, is an int64_t which cannot be
formatted using %x. This commit adjusts all misuses I was able to find
in the llvm-dwarfdump project.

Failing tests in https://reviews.llvm.org/D153800 lead to the discovery
and analysis of this issue.

Differential Revision: https://reviews.llvm.org/D155093