stdlib: annotate Unicode shims with nullability

Author: gribozavr

stdlib/public/SwiftShims/UnicodeShims.h

@@ -21,6 +21,10 @@
 #include "SwiftStdbool.h"
 #include "Visibility.h"
 
+#if __has_feature(nullability)
+#pragma clang assume_nonnull begin
+#endif
+
 #ifdef __cplusplus
 namespace swift { extern "C" {
 #endif
@@ -109,4 +113,8 @@ __swift_int32_t _swift_stdlib_unicode_strToLower(
 }} // extern "C", namespace swift
 #endif
 
+#if __has_feature(nullability)
+#pragma clang assume_nonnull end
+#endif
+
 #endif