[flang] Add semantic checks for threadprivate and declare target directives

This patch supports the following checks:
```
[5.1] 2.21.2 THREADPRIVATE Directive
The threadprivate directive must appear in the declaration section of
a scoping unit in which the common block or variable is declared.
[5.1] 2.14.7 Declare Target Directive
The directive must appear in the declaration section of a scoping unit
in which the common block or variable is declared.
```
Reviewed By: kiranchandramohan, shraiysh, NimishMishra

Differential Revision: https://reviews.llvm.org/D125767