[WoA][MSVC] Use default linker setting in MSVC-compatible driver [take 2]

At the moment "link.exe" is hard-coded as default linker in MSVC.cpp,
so there's no way to use LLD as default linker for MSVC driver.

This patch adds checking of CLANG_DEFAULT_LINKER to MSVC.cpp and
updates unit-tests that expect link.exe linker to explicitly select it
via -fuse-ld=link, so that buildbots and other builds that set
-DCLANG_DEFAULT_LINKER=foobar don't fail these tests.

This is a squash of
- https://reviews.llvm.org/D98493 (MSVC.cpp change) and
- https://reviews.llvm.org/D98862 (unit-tests change)

Reviewed By: maxim-kuvyrkov

Differential Revision: https://reviews.llvm.org/D98935