mysql/datetime: Improve TIME parsing logic

[dbussink]

In places inside the evalengine we need to know if a TIME instance was properly parsed, if a value could be extracted from input (but it wasn't entirely valid) or if there's enirely invalid input.

Before we only had one flag that indicated the strict parsing. So we couldn't really discern partial parsing success from entire failure. We tried that with checking for a zero time, but that's wrong. A zero time is still an entirely valid time and can be the result of successful partial parsing.

Related Issue(s)

This was identified in #15124, but the fix there didn't look like it was the right approach with yet another flag for allowing a zero time. This since we shouldn't really treat a zero time as special anyway.

Part of #15077

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[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (2c7b647) 70.62% compared to head (92f8b88) 70.65%.

Files	Patch %	Lines
go/mysql/datetime/parse.go	93.75%	3 Missing ⚠️
go/vt/sqlparser/literal.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15135      +/-   ##
==========================================
+ Coverage   70.62%   70.65%   +0.03%     
==========================================
  Files        1376     1376              
  Lines      182418   182454      +36     
==========================================
+ Hits       128828   128919      +91     
+ Misses      53590    53535      -55     

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[vmg]

I love this approach

[systay]

i < -math.MaxUint32 is no longer needed to check?

[dbussink]

@systay I realized it's a useless check, since when we get here we already consumed any - at the beginning. So it can never be negative here, so hence the change to parse it as uint and drop this bit.

[GrahamCampbell]

This is in the v19.0.0 milestone, but was merged to 20.x. Will this be backported?

[vmg]

@GrahamCampbell sure thing!