Push down ROUND(x) on decimal types (#2492)

[ti-srebot]

cherry-pick #2492 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tics repo:
git pr https://github.com/pingcap/tics/pull/2611

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tics.git pr/2611:release-5.1-4997809a0ad6

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What problem does this PR solve?

Issue Number: close #2426, part of #2550

Problem Summary: enable ROUND(x) on TiFlash.

What is changed and how it works?

What's Changed:

• IDataType.h , DataTypesNumber.h: add isFloatingPoint.
• DAGExpressionAnalyzer.cpp: construct tidbRound from tidbRoundWithFrac.
• DAGUtils.cpp: map TiPB codes to function names.
• FunctionsArithmetic.h, toUnsigned.h: move out toUnsigned for reusing in FunctionsRound.h.
• FunctionsRound.cpp: register new function.
• FunctionsRound.h: the implementation.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Unit test
• Integration test

Release note

• Support pushing down ROUND(X) to TiFlash.