[SYCL][Driver] Make /MD default with -fsycl for clang/clang++ drivers (#2480)

SYCL library is designed such a way that STL objects must cross the sycl.dll
boundary, which is guaranteed to work safe on Windows only if the runtime
in the app using sycl.dll and in sycl.dll is the same and is dynamic.

It is not possible to implement safe approach for using sycl libraries
built/linked with static C++ RT as it would cause having multiple copies
of C++ objects (such as scheduler, etc), which are supposed to be
singletones.

sycl.dll is built with /MD (linked to dynamic C++ RT), this is why this patch 
makes /MD default with -fsycl for clang and clang++ drivers.

Author: v-klochkov

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6258,6 +6258,19 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
       if (Args.hasFlag(options::OPT_fsycl_esimd, options::OPT_fno_sycl_esimd,
                        false))
         CmdArgs.push_back("-fsycl-explicit-simd");
+
+      if (!D.IsCLMode()) {
+        // SYCL library is guaranteed to work correctly only with dynamic
+        // MSVC runtime.
+        llvm::Triple AuxT = C.getDefaultToolChain().getTriple();
+        if (Args.hasFlag(options::OPT_fsycl_device_only, OptSpecifier(), false))
+          AuxT = llvm::Triple(llvm::sys::getProcessTriple());
+        if (AuxT.isWindowsMSVCEnvironment()) {
+          CmdArgs.push_back("-D_MT");
+          CmdArgs.push_back("-D_DLL");
+          CmdArgs.push_back("--dependent-lib=msvcrt");
+        }
+      }
     }
     if (IsSYCLOffloadDevice && JA.getType() == types::TY_SYCL_Header) {
       // Generating a SYCL Header
@@ -6836,8 +6849,7 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
     // Add SYCL dependent library
     if (Args.hasArg(options::OPT_fsycl) &&
         !Args.hasArg(options::OPT_nolibsycl)) {
-      if (RTOptionID == options::OPT__SLASH_MDd ||
-          RTOptionID == options::OPT__SLASH_MTd)
+      if (RTOptionID == options::OPT__SLASH_MDd)
         CmdArgs.push_back("--dependent-lib=sycld");
       else
         CmdArgs.push_back("--dependent-lib=sycl");

clang/lib/Driver/ToolChains/MSVC.cpp

@@ -370,8 +370,12 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
         Args.MakeArgString(std::string("-out:") + Output.getFilename()));
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles) &&
-      !C.getDriver().IsCLMode())
-    CmdArgs.push_back("-defaultlib:libcmt");
+      !C.getDriver().IsCLMode()) {
+    if (Args.hasArg(options::OPT_fsycl) && !Args.hasArg(options::OPT_nolibsycl))
+      CmdArgs.push_back("-defaultlib:msvcrt");
+    else
+      CmdArgs.push_back("-defaultlib:libcmt");
+  }
 
   if (!C.getDriver().IsCLMode() && !Args.hasArg(options::OPT_nostdlib) &&
       Args.hasArg(options::OPT_fsycl) && !Args.hasArg(options::OPT_nolibsycl)) {

clang/test/Driver/sycl-MD-default.cpp

@@ -1,5 +1,9 @@
 // REQUIRES: clang-driver
 
+// RUN: %clang -### -fsycl -c -target x86_64-unknown-windows-msvc %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-DEFAULT %s
+// RUN: %clangxx -### -fsycl -c -target x86_64-unknown-windows-msvc %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-DEFAULT %s
 // RUN: %clang_cl -### -fsycl -c %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=CHK-DEFAULT %s
 // RUN: %clang_cl -### -MD -fsycl -c %s 2>&1 \