New Error Prone 'IgnoredPureGetter' diagnostic

[cushon]

After 248ff0d, using Bazel and java_tools at head to build Bazel fails:

ERROR: external/com_google_protobuf/java/core/BUILD:112:13: Building external/com_google_protobuf/java/core/liblite_runtime_only.jar (89 source files) [for tool] failed: (Exit 1): java failed: error executing command (from target @com_google_protobuf//java/core:lite_runtime_only) external/remotejdk17_linux/bin/java '--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED' '--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED' ... (remaining 17 arguments skipped)
external/com_google_protobuf/java/core/src/main/java/com/google/protobuf/ArrayDecoders.java:785: error: [IgnoredPureGetter] Getters on AutoValues, AutoBuilders, and Protobuf Messages are side-effect free, so there is no point in calling them if the return value is ignored. While there are no side effects from the getter, the receiver may have side effects.
      ((GeneratedMessageLite.ExtendableMessage<?, ?>) message).ensureExtensionsAreMutable();
                                                                                         ^
    (see https://errorprone.info/bugpattern/IgnoredPureGetter)
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This diagnostic should probably not be an error.

[cushon]

3b57894