runtime: move the definition of report_bad_alloc_err

Use the reserved spelling for the weak attribute.  Move the definition into an
implementation file to avoid multiple definitions of the function from being
emitted.  PE/COFF does not support weak symbols, however, the runtime will never
link against LLVMSupport, so provide a single, strong definition of the
function.  Mark it as hidden visibility so that we do not expose it outside of
the runtime.

stdlib/public/runtime/LLVMSupport.cpp

@@ -0,0 +1,23 @@
+//===--- LLVMSupport.cpp - Swift Language ABI Metadata Support ------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+// ADT uses report_bad_alloc_error to report an error when it can't allocate
+// elements for a data structure. The swift runtime uses ADT without linking
+// against libSupport, so here we provide a stub to make sure we don't fail
+// to link.
+#if defined(swiftCore_EXPORTS)
+namespace llvm {
+void __attribute__((__weak__, __visibility__("hidden")))
+report_bad_alloc_error(const char *Reason, bool GenCrashDiag) {}
+} // end namespace llvm
+#endif
+