[SPARK-22699][SQL] GenerateSafeProjection should not use global variables for struct

[mgaido91]

What changes were proposed in this pull request?

GenerateSafeProjection is defining a mutable state for each struct, which is not needed. This is bad for the well known issues related to constant pool limits.
The PR replace the global variable with a local one.

How was this patch tested?

added UT

[mgaido91]

@cloud-fan @kiszk @viirya may you please review this? Thanks

[SparkQA]

Test build #84569 has finished for PR 19914 at commit 6d2ce8e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

Since GenerateSafeProjection produces a class, so I think it doesn't face such high pressure from constant pool limit. But it also looks no harm to replace this global variable with a local one.

[mgaido91]

@viirya it generates a class, but the changed code and therefore the additional variable is added for each struct object. It might be a problem especially for very complex schemas with nested structs.

[cloud-fan]

thanks, merging to master!