[foreign-abi] RFR: 8248480: Switch isVarArg layout attribute to Boolean

[Jorn Vernee]

Hi,

This trivial patch switches the type of the Windows var arg layout attribute from String to boolean.

Thanks,
Jorn

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Commit messages:
 - Change asVarArg attribute type from String to boolean

Changes: https://git.openjdk.java.net/panama-foreign/pull/231/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/231/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248480
  Stats: 3 lines in 2 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/231.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/231/head:pull/231

PR: https://git.openjdk.java.net/panama-foreign/pull/231