[ClangImporter] Pass -mcx16 to Clang invocation for x86_64 Linux targets

The Swift runtime and the concurrency module both use -mcx16, so there seems to be little point in not enabling it for Swift code in general.

This allows packages like swift-atomics to rely on double wide atomics always being available.

https://bugs.swift.org/browse/SR-13781

Author: lorentey

lib/ClangImporter/ClangImporter.cpp

@@ -781,6 +781,11 @@ importer::addCommonInvocationArguments(
     invocationArgStrs.push_back("-march=z13");
   }
 
+  if (triple.isOSLinux() && triple.getArch() == llvm::Triple::x86_64) {
+    // Enable double wide atomic intrinsics for x86_64 archs on Linux.
+    invocationArgStrs.push_back("-mcx16");
+  }
+
   if (!importerOpts.Optimization.empty()) {
     invocationArgStrs.push_back(importerOpts.Optimization);
   }