[Runtime] Add a declaration for swift_demangle to the header file and export it.

rdar://problem/20356017

Author: mikeash

include/swift/Demangling/Demangle.h

@@ -24,6 +24,7 @@
 #include <cassert>
 #include <cstdint>
 #include "llvm/ADT/StringRef.h"
+#include "swift/Runtime/Config.h"
 
 namespace llvm {
   class raw_ostream;
@@ -528,4 +529,30 @@ llvm::StringRef makeSymbolicMangledNameStringRef(const char *base);
 } // end namespace Demangle
 } // end namespace swift
 
+// NB: This function is not used directly in the Swift codebase, but is
+// exported for Xcode support and is used by the sanitizers. Please coordinate
+// before changing.
+//
+/// Demangles a Swift symbol name.
+///
+/// \param mangledName is the symbol name that needs to be demangled.
+/// \param mangledNameLength is the length of the string that should be
+/// demangled.
+/// \param outputBuffer is the user provided buffer where the demangled name
+/// will be placed. If nullptr, a new buffer will be malloced. In that case,
+/// the user of this API is responsible for freeing the returned buffer.
+/// \param outputBufferSize is the size of the output buffer. If the demangled
+/// name does not fit into the outputBuffer, the output will be truncated and
+/// the size will be updated, indicating how large the buffer should be.
+/// \param flags can be used to select the demangling style. TODO: We should
+//// define what these will be.
+/// \returns the demangled name. Returns nullptr if the input String is not a
+/// Swift mangled name.
+SWIFT_RUNTIME_EXPORT
+char *swift_demangle(const char *mangledName,
+                     size_t mangledNameLength,
+                     char *outputBuffer,
+                     size_t *outputBufferSize,
+                     uint32_t flags);
+
 #endif // SWIFT_DEMANGLING_DEMANGLE_H

stdlib/public/runtime/Demangle.cpp

@@ -639,23 +639,6 @@ swift::_swift_buildDemanglingForMetadata(const Metadata *type,
 // NB: This function is not used directly in the Swift codebase, but is
 // exported for Xcode support and is used by the sanitizers. Please coordinate
 // before changing.
-//
-/// Demangles a Swift symbol name.
-///
-/// \param mangledName is the symbol name that needs to be demangled.
-/// \param mangledNameLength is the length of the string that should be
-/// demangled.
-/// \param outputBuffer is the user provided buffer where the demangled name
-/// will be placed. If nullptr, a new buffer will be malloced. In that case,
-/// the user of this API is responsible for freeing the returned buffer.
-/// \param outputBufferSize is the size of the output buffer. If the demangled
-/// name does not fit into the outputBuffer, the output will be truncated and
-/// the size will be updated, indicating how large the buffer should be.
-/// \param flags can be used to select the demangling style. TODO: We should
-//// define what these will be.
-/// \returns the demangled name. Returns nullptr if the input String is not a
-/// Swift mangled name.
-SWIFT_RUNTIME_EXPORT
 char *swift_demangle(const char *mangledName,
                      size_t mangledNameLength,
                      char *outputBuffer,