[DebugInfo][Docs] Document variable location metadata transformations

This patch adds documentation explaining how variable location information is
compiled from the IR representation down to the end of the codegen pipeline,
but avoiding discussion of file formats and encoding.

This should make it clearer how the dbg.value / dbg.declare etc intrinsics
are transformed and arranged into DBG_VALUE instructions, and their meaning.

Differential Revision: https://reviews.llvm.org/D59790

llvm-svn: 358385