[Preprocessor] Ensure newline after #pragma introduced by -fms-extensions.

The -fms-extensions converts __pragma (and _Pragma) into a #pragma that
has to occur at the beginning of a line and end with a newline. This
patch ensures that the newline after the #pragma is added even if
Token::isAtStartOfLine() indicated that we should not start a newline.

Committing relying post-commit review since the change is small, some
downstream uses might be blocked without this fix, and to make clear the
decision of the new -fminimize-whitespace feature (fix on main, revert
on clang-13.x branch) suggested by @aaron.ballman in D104601.

Differential Revision: https://reviews.llvm.org/D107183