[flang] SAVE statement should not apply to nested scoping units

SAVE statement, according to 8.6.14, must apply to the same scoping
unit, that excludes nested scoping units. For example, if the SAVE
statement is found in a MODULE, the functions contained in that module
should not inherit the SAVE attribute. I think that the code was doing
this, failing the following source:

```
MODULE pippo
SAVE

CONTAINS
PURE FUNCTION fft_stick_index( )
   IMPLICIT NONE
   INTEGER :: fft_stick_index
   INTEGER :: mc  !error: A pure subprogram may not have a variable with the SAVE attribute
END FUNCTION

END MODULE
```

Differential Revision: https://reviews.llvm.org/D88279