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