[lld/coff] Make lld-link work in a non-MSVC shell, add /winsysroot:

Makes lld-link work in a non-MSVC shell by autodetecting MSVC toolchain. Also
adds support for /winsysroot and a few other switches.

All this is done by refactoring to share code with clang-cl's existing support
for the same.

Differential Revision: https://reviews.llvm.org/D118070

Author: pkasting

clang/docs/tools/clang-formatted-files.txt

@@ -285,7 +285,6 @@ clang/lib/Driver/ToolChains/InterfaceStubs.h
 clang/lib/Driver/ToolChains/Minix.h
 clang/lib/Driver/ToolChains/MipsLinux.cpp
 clang/lib/Driver/ToolChains/MSP430.h
-clang/lib/Driver/ToolChains/MSVCSetupApi.h
 clang/lib/Driver/ToolChains/PPCFreeBSD.cpp
 clang/lib/Driver/ToolChains/PPCFreeBSD.h
 clang/lib/Driver/ToolChains/PPCLinux.h
@@ -5150,6 +5149,8 @@ llvm/include/llvm/Support/MemoryBufferRef.h
 llvm/include/llvm/Support/MSP430AttributeParser.h
 llvm/include/llvm/Support/MSP430Attributes.h
 llvm/include/llvm/Support/MSVCErrorWorkarounds.h
+llvm/include/llvm/Support/MSVCPaths.h
+llvm/include/llvm/Support/MSVCSetupApi.h
 llvm/include/llvm/Support/Parallel.h
 llvm/include/llvm/Support/PGOOptions.h
 llvm/include/llvm/Support/PointerLikeTypeTraits.h
@@ -5840,6 +5841,7 @@ llvm/lib/Support/Memory.cpp
 llvm/lib/Support/MemoryBufferRef.cpp
 llvm/lib/Support/MSP430AttributeParser.cpp
 llvm/lib/Support/MSP430Attributes.cpp
+llvm/lib/Support/MSVCPaths.cpp
 llvm/lib/Support/Optional.cpp
 llvm/lib/Support/Parallel.cpp
 llvm/lib/Support/Program.cpp