some optimizations may cause some wrong result for extension functions.

[lcwangchao]

Bug Report

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1. Minimal reproduce step (Required)

We found serval problems when a extension function is optimized by planner:

• extension function fold by mistake even if it has some side effect.
• extension function result may be cached in some expression such as regexp
• extension function maybe cached in plan cache that will cause privilege check pass even if some privileges are revoked.

You can see the test: https://github.com/pingcap/tidb/pull/51926/files#diff-b06ca4f8065e5716bc7b3e5d5cfe389156ad9125bddb40ff1d95a59d889d45c8R461 for detail

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)