DWARF64 Fixes

1. DW_FORM_strp and DW_FORM_sec_offset are 64bits for DWARF64 / 32bits for DWARF32
They are different from DW_FORM_addr, whose size is specified in .debug_info

2. Bump DWARF version support form [2,3] to [2,4] in DWARFDebugLine.cpp

3. Fix DWARFDebugLine to support DWARF64

See http://reviews.llvm.org/D5307 for more details.
Reviewed by Greg Clayton and Jason Molenda.

Change by Tong Shen.

llvm-svn: 217607