[clang][flang][windows] Prefer user-provided library paths (-L)

Currently the paths to compiler-rt and the Flang runtimes from the LLVM
build/install directory are preferred over any user-provided library
paths. This means a user can't override compiler-rt or the Flang
runtimes with custom versions.

This patch changes the link order to prefer library paths specified
with -L over the LLVM paths. This matches the behaviour of clang
and flang on Linux.

Author: DavidTruby

clang/lib/Driver/ToolChains/MSVC.cpp

@@ -134,6 +134,10 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
           Args.MakeArgString(std::string("-libpath:") + WindowsSdkLibPath));
   }
 
+  if (!C.getDriver().IsCLMode() && Args.hasArg(options::OPT_L))
+    for (const auto &LibPath : Args.getAllArgValues(options::OPT_L))
+      CmdArgs.push_back(Args.MakeArgString("-libpath:" + LibPath));
+
   if (C.getDriver().IsFlangMode()) {
     addFortranRuntimeLibraryPath(TC, Args, CmdArgs);
     addFortranRuntimeLibs(TC, Args, CmdArgs);
@@ -154,10 +158,6 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (TC.getVFS().exists(CRTPath))
     CmdArgs.push_back(Args.MakeArgString("-libpath:" + CRTPath));
 
-  if (!C.getDriver().IsCLMode() && Args.hasArg(options::OPT_L))
-    for (const auto &LibPath : Args.getAllArgValues(options::OPT_L))
-      CmdArgs.push_back(Args.MakeArgString("-libpath:" + LibPath));
-
   CmdArgs.push_back("-nologo");
 
   if (Args.hasArg(options::OPT_g_Group, options::OPT__SLASH_Z7))