Merge pull request #6138 from hughbe/shims-msvc

Fix errors and warnings building swift/shims on Windows using MSVC

Author: compnerd

stdlib/public/SwiftShims/LibcShims.h

@@ -35,7 +35,7 @@ namespace swift { extern "C" {
 // the current platform in the runtime code.
 #if defined(__linux__) && defined (__arm__) && !defined(__android__)
 typedef           int __swift_ssize_t;
-#elif defined(_MSC_VER)
+#elif defined(_WIN32)
 #if defined(_M_ARM) || defined(_M_IX86)
 typedef           int __swift_ssize_t;
 #elif defined(_M_X64)

stdlib/public/SwiftShims/Visibility.h

@@ -18,7 +18,11 @@
 #ifndef SWIFT_STDLIB_SHIMS_VISIBILITY_H
 #define SWIFT_STDLIB_SHIMS_VISIBILITY_H
 
-#if defined(__has_feature) && __has_feature(nullability)
+#if !defined(__has_feature)
+#define __has_feature(x) false
+#endif
+
+#if __has_feature(nullability)
 // Provide macros to temporarily suppress warning about the use of
 // _Nullable and _Nonnull.
 # define SWIFT_BEGIN_NULLABILITY_ANNOTATIONS                        \