[jdk17] RFR: JDK-8263321: Regression 8% in javadoc-steady in 17-b11

[Hannes Wallnöfer]

This change fixes a performance regression caused by passing instances of `com.sun.tools.javac.util.Name` to a `html.markup.Text` object, where their content is accessed using`CharSequence` methods which are implemented very inefficiently. 

Since improving `javac.util.Name` will likely require a bit more work (see JDK-8268622), a safe short term solution for JDK 17 is to make sure instances of `Name` are converted to `String` before invoking said `CharSequence` methods. The main fix is to convert to `String` early in `Entity.escapeHtmlChars(CharSequence)` which is invoked by the `Text` constructor. There are a few more `.toString()` invocations added in places where a `Name` is appended to a `StringBuilder`, although these have less performance impact.

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Commit messages:
 - JDK-8263321: Regression 8% in javadoc-steady in 17-b11

Changes: https://git.openjdk.java.net/jdk17/pull/42/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=42&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263321
  Stats: 21 lines in 7 files changed: 2 ins; 0 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/42.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/42/head:pull/42

PR: https://git.openjdk.java.net/jdk17/pull/42