Add C++ dependencies to xray runtime

[dberris]

Summary

Depends on D21982 which implements the in-memory logging implementation of the
XRay runtime. These additional changes also depends on D20352 which adds the
bulk of XRay flags/dependencies when using the -fxray-instrument flag from
Clang.

[dberris]

• Check first whether D is actually not nullptr

[dberris]

Rebase

[aaron.ballman]

LGTM (once other deps land).

[dberris]

Rebase

[dberris]

• Undo unnecessary formatting change

lib/Driver/Tools.cpp

if (Args.hasArg(options::OPT_fxray_instrument,
     CmdArgs.push_back("-whole-archive");
     CmdArgs.push_back(TC.getCompilerRTArgString(Args, "xray", false));
     CmdArgs.push_back("-no-whole-archive");
     return true;
   }
   return false;
 }
 
-static void linkXRayRuntimeDeps(const ToolChain &TC, ArgStringList &CmdArgs) {
+static void linkXRayRuntimeDeps(const ToolChain &TC, const ArgList &Args,
+                                ArgStringList &CmdArgs) {
   CmdArgs.push_back("--no-as-needed");
   CmdArgs.push_back("-lpthread");
   CmdArgs.push_back("-lrt");
   CmdArgs.push_back("-lm");
   CmdArgs.push_back("-latomic");
+  if (TC.GetCXXStdlibType(Args) == ToolChain::CST_Libcxx)
+    CmdArgs.push_back("-lc++");
+  else
+    CmdArgs.push_back("-lstdc++");
   if (TC.getTriple().getOS() != llvm::Triple::FreeBSD)
     CmdArgs.push_back("-ldl");
 }
 
 static bool areOptimizationsEnabled(const ArgList &Args) {
   // Find the last -O arg and see if it is non-zero.
   if (Arg *A = Args.getLastArg(options::OPT_O_Group))
     return !A->getOption().matches(options::OPT_O0);
if (!Args.hasArg(options::OPT_nostdlib)) {
     if (!Args.hasArg(options::OPT_nodefaultlibs)) {
       if (Args.hasArg(options::OPT_static))
         CmdArgs.push_back("--start-group");
 
       if (NeedsSanitizerDeps)
         linkSanitizerRuntimeDeps(ToolChain, CmdArgs);
 
       if (NeedsXRayDeps)
-        linkXRayRuntimeDeps(ToolChain, CmdArgs);
+        linkXRayRuntimeDeps(ToolChain, Args, CmdArgs);
 
       bool WantPthread = Args.hasArg(options::OPT_pthread) ||
                          Args.hasArg(options::OPT_pthreads);
 
       if (Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
                        options::OPT_fno_openmp, false)) {
         // OpenMP runtimes implies pthreads when using the GNU toolchain.
         // FIXME: Does this really make sense for all GNU toolchains?