Make a function to correctly extract the DW_AT_high_pc given the low pc value.

DWARF 4 and later supports encoding the PC as an address or as as offset from the low PC. Clients using DWARFDie should be insulated from how to extract the high PC value. This function takes care of extracting the form value and looking for the correct form.

Differential Revision: https://reviews.llvm.org/D27885

llvm-svn: 290131