[flang][msvc] Fix lambda capture ambiguity. NFC.

Patch D88695 introduces a new local variable inside a lambda with the same name as a variable outside of it. In some of the if constexpr regions, msvc prioritizes the outer declaration and emits the error.
```
C:\Users\meinersbur\src\llvm-project\flang\lib\Evaluate\fold-implementation.h(1200): error C3493: 'context' cannot be implicitly captured because no default capture mode has been specified
```

This is fixed by giving the inner variable a different name.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D89367