[LiveDebugValues] Add switches for using instr-ref variable locations

This patch adds the -Xclang option
"-fexperimental-debug-variable-locations" and same LLVM CodeGen option,
to pick which variable location tracking solution to use.

Right now all the switch does is pick which LiveDebugValues
implementation to use, the normal VarLoc one or the instruction
referencing one in rGae6f78824031. Over time, the aim is to add fragments
of support in aid of the value-tracking RFC:

  http://lists.llvm.org/pipermail/llvm-dev/2020-February/139440.html

also controlled by this command line switch. That will slowly move
variable locations to be defined by an instruction calculating a value,
and a DBG_INSTR_REF instruction referring to that value. Thus, this is
going to grow into a "use the new kind of variable locations" switch,
rather than just "use the new LiveDebugValues implementation".

Differential Revision: https://reviews.llvm.org/D83048