Extend the DWARFExpression address handling to support 16-bit addresses

This allows the DWARFExpression class to handle addresses without
crashing on targets with 16-bit pointers like AVR.

This is required in order to generate assembly from clang via the '-S'
flag.

This fixes an error with the following message:

clang: llvm/include/llvm/DebugInfo/DWARF/DWARFExpression.h:132: llvm::DWARFExpression::DWARFExpression(llvm::DataExtractor, uint16_t, uint8_t):
       Assertion `AddressSize == 8 || AddressSize == 4' failed.
llvm-svn: 362290