stage2: don't depend on windows SDK C++ code when unavailable

BRANCH_TODO

@@ -46,3 +46,6 @@
  * there are a couple panic("TODO") in clang options parsing
  * std.testing needs improvement to support exposing directory path for its tmp dir (look for "bogus")
  * integrate target features into building assembly code
+ * libc_installation.zig: make it look for msvc only if msvc abi is chosen
+ * switch the default C ABI for windows to be mingw-w64
+ * port windows_sdk.cpp to zig

src-self-hosted/libc_installation.zig

@@ -169,6 +169,8 @@ pub const LibCInstallation = struct {
         var self: LibCInstallation = .{};
 
         if (is_windows) {
+            if (!build_options.have_llvm)
+                return error.WindowsSdkNotFound;
             var sdk: *ZigWindowsSDK = undefined;
             switch (zig_find_windows_sdk(&sdk)) {
                 .None => {