Add casts to avoid warnings about implicit conversions losing precision

This fixes warnings like these:

DwarfInstructions.hpp:85:25: warning: implicit conversion
      loses integer precision: 'uint64_t' (aka 'unsigned long long') to
      'libunwind::DwarfInstructions<libunwind::LocalAddressSpace,
      libunwind::Registers_arm>::pint_t' (aka 'unsigned int')
      [-Wshorten-64-to-32]

DwarfInstructions.hpp:88:25: warning: implicit conversion
      loses integer precision: 'uint64_t' (aka 'unsigned long long') to
      'libunwind::DwarfInstructions<libunwind::LocalAddressSpace,
      libunwind::Registers_arm>::pint_t' (aka 'unsigned int')
      [-Wshorten-64-to-32]

Differential Revision: https://reviews.llvm.org/D56983

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@351877 91177308-0d34-0410-b5e6-96231b3b80d8