DWARFExpression: Convert file addresses to load addresses early on.

This is a change that only affects Swift and is NFC for the language
plugins on llvm.org. In Swift, we can have global variables with a
location such as DW_OP_addr <addr> DW_OP_deref. The DWARF expression
evaluator doesn't know how to apply a DW_OP_deref to a file address,
but at the very end we convert the file address into a load address.

This patch moves the file->load address conversion to right after the
result of the DW_OP_addr is pushed onto the stack so that a subsequent
DW_OP_deref (and potentially other operations) can be interpreted.

rdar://problem/39767528

Differential revision: https://reviews.llvm.org/D46362

llvm-svn: 331462