[lworld] RFR: JDK-8269023 [lworld] L/Q<type> support

[David Simms]

Merge work from lqagain branch

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Commit messages:
 - 8268981: [lworld] UnifiedPrimitiveClassNestHostTest.java should test with the primary mirror
 - 8268980: [lword] Fix Class::descriptorString to return L-descriptor for primitive ref type
 - Merge lworld
 - 8268908: Update test/jdk/valhalla/valuetypes tests to prepare for jtreg 6 and other clean up
 - 8268656: [lqagain] A few code chunks dealing with dual class scheme still left in
 - 8268358: [lworld] toString for primitive class should return `ClassName at hash`
 - 8268584: [lqagain] Javac tests should not needlessly hardcode toString() output
 - 8267597: [lworld] Withdraw all support for bifurcated class generation for primitive classes
 - 8268418: [lworld] VM assert illegal mirror klass when calling Class::getInterfaces on a flat array
 - 8268399: [BACKOUT] [lworld] Tests must be update after core reflection changes for the L/Q model
 - ... and 12 more: https://git.openjdk.java.net/valhalla/compare/85af6b56...521bed27

Changes: https://git.openjdk.java.net/valhalla/pull/453/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=453&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269023
  Stats: 4484 lines in 188 files changed: 1629 ins; 2014 del; 841 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/453.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/453/head:pull/453

PR: https://git.openjdk.java.net/valhalla/pull/453