[DebugInfo][InstrRef][NFC] Use depth-first scope search for variable locs

This patch aims to reduce max-rss from instruction referencing, by avoiding
keeping variable value information in memory for too long. Instead of
computing all the variable values then emitting them to DBG_VALUE
instructions, this patch tries to stream the information out through a
depth first search:
 * Make use of the fact LexicalScopes gives a depth-number to each lexical
   scope,
 * Produce a map that identifies the last lexical scope to make use of a
   block,
 * Enumerate each scope in LexicalScopes' DFS order, solving the variable
   value problem,
 * After each scope is processed, look for any blocks that won't be used by
   any other scope, and emit all the variable information to DBG_VALUE
   instructions.

Differential Revision: https://reviews.llvm.org/D118460

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