[MinGW] Link to correct openmp library

[mstorsjo]

Please add a testcase in test/Driver/, it looks like fopenmp.c there might be a good target to add a few lines in.

[SquallATF]

• add test

[mstorsjo]

LGTM, I can commit it for you on Monday, after giving others a chance to comment.

lib/Driver/ToolChains/MinGW.cpp

if (!Args.hasArg(options::OPT_nodefaultlibs)) {
         CmdArgs.push_back("--start-group");
 
       if (Args.hasArg(options::OPT_fstack_protector) ||
           Args.hasArg(options::OPT_fstack_protector_strong) ||
           Args.hasArg(options::OPT_fstack_protector_all)) {
         CmdArgs.push_back("-lssp_nonshared");
         CmdArgs.push_back("-lssp");
       }
-      if (Args.hasArg(options::OPT_fopenmp))
+
+      if (Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
+                       options::OPT_fno_openmp, false)) {
+        switch (TC.getDriver().getOpenMPRuntime(Args)) {
+        case Driver::OMPRT_OMP:
+          CmdArgs.push_back("-lomp");
+          break;
+        case Driver::OMPRT_IOMP5:
+          CmdArgs.push_back("-liomp5md");
+          break;
+        case Driver::OMPRT_GOMP:
           CmdArgs.push_back("-lgomp");
+          break;
+        case Driver::OMPRT_Unknown:
+          // Already diagnosed.
+          break;
+        }
+      }
 
       AddLibGCC(Args, CmdArgs);
 
       if (Args.hasArg(options::OPT_pg))
         CmdArgs.push_back("-lgmon");
 
       if (Args.hasArg(options::OPT_pthread))
         CmdArgs.push_back("-lpthread");