NFC: simplify Darwin environment handling

[jfb]

Summary

The previous code detected conflicts through copy-pasta, this versions
uses a 'loop'.

[arphaman]

I think we might need to run it past the internal builds to see if anything breaks.

[jfb]

In D55895#1336777, @arphaman wrote:

I think we might need to run it past the internal builds to see if anything breaks.

We've been running it for a while without breakage. Good to go?

[arphaman]

LGTM

[jfb]

Rebase

lib/Driver/ToolChains/Darwin.cpp

getDeploymentTargetFromEnvironmentVariables(const Driver &TheDriver,
   };
   static_assert(llvm::array_lengthof(EnvVars) == Darwin::LastDarwinPlatform + 1,
                 "Missing platform");
   for (const auto &I : llvm::enumerate(llvm::makeArrayRef(EnvVars))) {
     if (char *Env = ::getenv(I.value()))
       Targets[I.index()] = Env;
   }
 
-  // Do not allow conflicts with the watchOS target.
-  if (!Targets[Darwin::WatchOS].empty() &&
-      (!Targets[Darwin::IPhoneOS].empty() || !Targets[Darwin::TvOS].empty())) {
-    TheDriver.Diag(diag::err_drv_conflicting_deployment_targets)
-        << "WATCHOS_DEPLOYMENT_TARGET"
-        << (!Targets[Darwin::IPhoneOS].empty() ? "IPHONEOS_DEPLOYMENT_TARGET"
-                                               : "TVOS_DEPLOYMENT_TARGET");
-  }
-
-  // Do not allow conflicts with the tvOS target.
-  if (!Targets[Darwin::TvOS].empty() && !Targets[Darwin::IPhoneOS].empty()) {
-    TheDriver.Diag(diag::err_drv_conflicting_deployment_targets)
-        << "TVOS_DEPLOYMENT_TARGET"
-        << "IPHONEOS_DEPLOYMENT_TARGET";
-  }
-
   // Allow conflicts among OSX and iOS for historical reasons, but choose the
   // default platform.
   if (!Targets[Darwin::MacOS].empty() &&
       (!Targets[Darwin::IPhoneOS].empty() ||
        !Targets[Darwin::WatchOS].empty() || !Targets[Darwin::TvOS].empty())) {
     if (Triple.getArch() == llvm::Triple::arm ||
         Triple.getArch() == llvm::Triple::aarch64 ||
         Triple.getArch() == llvm::Triple::thumb)
       Targets[Darwin::MacOS] = "";
     else
       Targets[Darwin::IPhoneOS] = Targets[Darwin::WatchOS] =
           Targets[Darwin::TvOS] = "";
+  } else {
+    // Don't allow conflicts in any other platform.
+    int FirstTarget = llvm::array_lengthof(Targets);
+    for (int I = 0; I != llvm::array_lengthof(Targets); ++I) {
+      if (Targets[I].empty())
+        continue;
+      if (FirstTarget == llvm::array_lengthof(Targets))
+        FirstTarget = I;
+      else
+        TheDriver.Diag(diag::err_drv_conflicting_deployment_targets)
+            << Targets[FirstTarget] << Targets[I];
+    }
   }
 
   for (const auto &Target : llvm::enumerate(llvm::makeArrayRef(Targets))) {
     if (!Target.value().empty())
       return DarwinPlatform::createDeploymentTargetEnv(
           (Darwin::DarwinPlatformKind)Target.index(), EnvVars[Target.index()],
           Target.value());
   }