RFR: 8276173: Clean up and remove unneeded casts in HeapDumper

[Leo Korinth]

HeapDumper does a lot of unneeded casts. Some arguments should be const. Headers are not correctly sorted. Comment about identifier size on Windows and Solaris is not true.

First I cleaned up casting in the "union casting", but then I decided it was better to create a temporary bit_cast that we can use until we get the proper one in c++20.

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Commit messages:
 - 8276173: Clean up and remove unneeded casts in HeapDumper

Changes: https://git.openjdk.java.net/jdk/pull/6211/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6211&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276173
  Stats: 53 lines in 1 file changed: 8 ins; 14 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6211.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6211/head:pull/6211

PR: https://git.openjdk.java.net/jdk/pull/6211