RFR: 8278965: crash in SymbolTable::do_lookup [v2]

[Coleen Phillimore]

> Please review this small change to hold the refcount of a super class name while calling handle_parallel_super_load. Contrary to what I wrote in JDK-8298061, the refcount in the Placeholder table should never go to one.  I can reproduce this with btree012 occasionally, so this fixes this too.
> 
> The include in placeholders.hpp is required to make the gtest compiled since placeholders header file has Symbol::maybe_increment_refcount and decrement calls.
> 
> Tested with the graal test that failed, and tier1-7.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Fix variable name.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12491/files
  - new: https://git.openjdk.org/jdk/pull/12491/files/3f2bea79..90f0d989

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12491&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12491&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12491.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12491/head:pull/12491

PR: https://git.openjdk.org/jdk/pull/12491