[flang][msvc] Add explicit function template argument to applyFunction. NFC.

Msvc has difficulties deducing the template argument here. The error message is:
```
basic-parsers.h(790,12): error C2672: 'applyFunction': no matching overloaded function found
```
Explicitly pass the first template argument to help it.

This patch is part of the series to make flang compilable with MS Visual Studio <http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html>.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D87961