Resubmit: [Salvage] Change salvage debug info implementation to use DW_OP_LLVM_convert where needed

Fixes issue: https://bugs.llvm.org/show_bug.cgi?id=40645

Previously, LLVM had no functional way of performing casts inside of a
DIExpression(), which made salvaging cast instructions other than Noop casts
impossible. With the recent addition of DW_OP_LLVM_convert this salvaging is
now possible, and so can be used to fix the attached bug as well as any cases
where SExt instruction results are lost in the debugging metadata. This patch
introduces this fix by expanding the salvage debug info method to cover these
cases using the new operator.

Differential revision: https://reviews.llvm.org/D61184

llvm-svn: 360902