RFR: Move back BacktraceBuilder

[Coleen Phillimore]

Just code motion to help with the review.
Tested with build fastdebug, product and loom-tier1 to loom-tier3 (in progress)

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Commit messages:
 - Move back BacktraceBuilder

Changes: https://git.openjdk.java.net/loom/pull/115/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=115&range=00
  Stats: 445 lines in 1 file changed: 223 ins; 222 del; 0 mod
  Patch: https://git.openjdk.java.net/loom/pull/115.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/115/head:pull/115

PR: https://git.openjdk.java.net/loom/pull/115