planner: allow queries on virtual columns to use index

[qw4990]

What problem does this PR solve?

Fix #5189, allow queries on virtual columns to use index.

What is changed and how it works?

Before this PR, virtual columns are replaced with corresponding physical columns when building the basic logical plan, which happens before the optimization.

Therefore, when the optimizer gets the replaced logical plan, there is no virtual column in it, so the optimizer will not use the corresponding virtual index to optimize this plan.

In this PR, we postpone replacing the virtual column until the optimization is finished, so the optimizer will take virtual columns into account and virtual indices will be used.

Check List

Tests

• Unit test

[codecov]

Codecov Report

Merging #10802 into master will decrease coverage by 0.0052%.
The diff coverage is 73.7931%.

@@               Coverage Diff                @@
##             master     #10802        +/-   ##
================================================
- Coverage   81.2832%   81.2779%   -0.0053%     
================================================
  Files           424        424                
  Lines         91004      91096        +92     
================================================
+ Hits          73971      74041        +70     
- Misses        11704      11726        +22     
  Partials       5329       5329

[qw4990]

/run-all-tests

[qw4990]

Waiting for https://github.com/pingcap/tidb-test/pull/841;

[qw4990]

PTAL @lamxTyler @winoros @crazycs520

[alivxxx]

LGTM

[qw4990]

Some new problems and corner cases are found when a virtual column is in the inner child of an IndexJoin or an IndexLookup, which are hard to fix since it will make our code weird and hard to maintain.
Therefore we decide to close this PR and fix this virtual column problem after finishing our cascades planner.