RFR: 8257676: Simplify WeakProcessorPhase

[Kim Barrett]

Please review this reimplementation of WeakProcessorPhase.  It is changed to
a scoped enum at namespace scope, and uses the recently added EnumIterator
facility to provide iteration, rather than a bespoke iterator class.

This is a step toward eliminating it entirely.  I've split it out into a
separate PR to make the review of the follow-up work a bit easier.

As part of this the file weakProcessorPhases.hpp is renamed to
weakProcessorPhase.hpp, but git doesn't seem to be recognizing that as a
rename and (majorly) edit, instead treating it as a remove and add a new
file.

Testing: mach5 tier1

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Commit messages:
 - simplify phases and use enum class

Changes: https://git.openjdk.java.net/jdk/pull/1620/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1620&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257676
  Stats: 219 lines in 8 files changed: 38 ins; 171 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1620.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1620/head:pull/1620

PR: https://git.openjdk.java.net/jdk/pull/1620