[Clang mangling] Don't mangle when Clang says not to

We have been mangling extern "C" symbols when building with C++
interoperability, leading to incorrectly-mangled names such as
_Z6memset that should have been unmangled "memset". Fix this so we get
consistent mangling between C and C++ interoperability modes.

Fixes rdar://164495210.

test/Inputs/clang-importer-sdk/usr/include/string.h

@@ -7,6 +7,12 @@
 
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+
+typedef __SIZE_TYPE__ size_t;
+#endif
+
 void* memcpy(void* s1, const void* s2, size_t n);
 void* memmove(void* s1, const void* s2, size_t n);
 char* strcpy (char* s1, const char* s2);
@@ -30,4 +36,8 @@ void* memset(void* s, int c, size_t n);
 char* strerror(int errnum);
 size_t strlen(const char* s);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // SDK_STRING_H