[Backtracing] Fix runtime symbols.

The runtime functions must be marked `extern "C"` because the symbols
are not mangled when exported.

rdar://137201928

Author: al45tair

stdlib/public/Backtracing/Runtime.swift

@@ -19,11 +19,6 @@ import Swift
 
 internal import BacktracingImpl.Runtime
 
-let swift_reportWarning = swift.runtime.swift_reportWarning
-let _swift_backtrace_isThunkFunction
-  = swift.runtime._swift_backtrace_isThunkFunction
-let _swift_backtrace_demangle = swift.runtime._swift_backtrace_demangle
-
 typealias CrashInfo = swift.runtime.backtrace.CrashInfo
 typealias memserver_req = swift.runtime.backtrace.memserver_req
 typealias memserver_resp = swift.runtime.backtrace.memserver_resp

stdlib/public/Backtracing/modules/Runtime/Runtime.h

@@ -23,25 +23,21 @@
 #include "swift/Runtime/CrashInfo.h"
 
 #ifdef __cplusplus
-namespace swift {
-namespace runtime {
+#define EXTERN_C extern "C"
+#else
+#define EXTERN_C
 #endif
 
 // Can't import swift/Runtime/Debug.h because it assumes C++
-void swift_reportWarning(uint32_t flags, const char *message);
+EXTERN_C void swift_reportWarning(uint32_t flags, const char *message);
 
 // Returns true if the given function is a thunk function
-bool _swift_backtrace_isThunkFunction(const char *rawName);
+EXTERN_C bool _swift_backtrace_isThunkFunction(const char *rawName);
 
 // Demangle the given raw name (supports Swift and C++)
-char *_swift_backtrace_demangle(const char *rawName,
-                                size_t rawNameLength,
-                                char *outputBuffer,
-                                size_t *outputBufferSize);
-
-#ifdef __cplusplus
-} // namespace runtime
-} // namespace swift
-#endif
+EXTERN_C char *_swift_backtrace_demangle(const char *rawName,
+                                         size_t rawNameLength,
+                                         char *outputBuffer,
+                                         size_t *outputBufferSize);
 
 #endif // SWIFT_BACKTRACING_RUNTIME_H