[Syntax] Test both the default and windows target platforms in unittests

Summary:
This increases the coverage for things that differ between Linux and Windows, such as `-fdelayed-template-parsing`. This would have prevented the rollback of https://reviews.llvm.org/D76346.

While at it, update -std=c++11 to c++17 for the test.

Reviewers: gribozavr2

Reviewed By: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76433