## What changes were proposed in this pull request?
Here is the problem description from the JIRA.
```
When the inputs contain the constant 'infinity', Spark SQL does not generate the expected results.
SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
FROM (VALUES ('1'), (CAST('infinity' AS DOUBLE))) v(x);
SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
FROM (VALUES ('infinity'), ('1')) v(x);
SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
FROM (VALUES ('infinity'), ('infinity')) v(x);
SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
FROM (VALUES ('-infinity'), ('infinity')) v(x);
The root cause: Spark SQL does not recognize the special constants in a case insensitive way. In PostgreSQL, they are recognized in a case insensitive way.
Link: https://www.postgresql.org/docs/9.3/datatype-numeric.html
```
In this PR, the casting code is enhanced to handle these `special` string literals in case insensitive manner.
## How was this patch tested?
Added tests in CastSuite and modified existing test suites.
Closes#25331 from dilipbiswal/double_infinity.
Authored-by: Dilip Biswal <dbiswal@us.ibm.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
331f2657d9