[SPARK-16714][SPARK-16735][SPARK-16646] array, map, greatest, least's type coercion should handle decimal type

## What changes were proposed in this pull request?

Here is a table about the behaviours of `array`/`map` and `greatest`/`least` in Hive, MySQL and Postgres:

|    |Hive|MySQL|Postgres|
|---|---|---|---|---|
|`array`/`map`|can find a wider type with decimal type arguments, and will truncate the wider decimal type if necessary|can find a wider type with decimal type arguments, no truncation problem|can find a wider type with decimal type arguments, no truncation problem|
|`greatest`/`least`|can find a wider type with decimal type arguments, and truncate if necessary, but can't do string promotion|can find a wider type with decimal type arguments, no truncation problem, but can't do string promotion|can find a wider type with decimal type arguments, no truncation problem, but can't do string promotion|

I think these behaviours makes sense and Spark SQL should follow them.

This PR fixes `array` and `map` by using `findWiderCommonType` to get the wider type.
This PR fixes `greatest` and `least` by add a `findWiderTypeWithoutStringPromotion`, which provides similar semantic of `findWiderCommonType`, but without string promotion.

## How was this patch tested?

new tests in `TypeCoersionSuite`

Author: Wenchen Fan <wenchen@databricks.com>
Author: Yin Huai <yhuai@databricks.com>

Closes #14439 from cloud-fan/bug.

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