[Driver] Adjust Mac Catalyst deployment target to >= iOS 13 for the linker

Mac Catalyst was introduced with an iOS deployment target of 13.0.
If given a deployment target before that, adjust the deployment target
to 13.0 for the linker.

Author: DougGregor

lib/Driver/DarwinToolChains.cpp

@@ -520,6 +520,14 @@ toolchains::Darwin::addDeploymentTargetArgs(ArgStringList &Arguments,
     unsigned major, minor, micro;
     if (tripleIsMacCatalystEnvironment(triple)) {
       triple.getiOSVersion(major, minor, micro);
+
+      // Mac Catalyst was introduced with an iOS deployment target of 13.0;
+      // the linker doesn't want to see a deployment target before that.
+      if (major < 13) {
+        major = 13;
+        minor = 0;
+        micro = 0;
+      }
     } else {
       switch (getDarwinPlatformKind((triple))) {
       case DarwinPlatformKind::MacOS:

test/Driver/macabi-environment.swift

@@ -17,6 +17,22 @@
 // IOS13-MACABI-DAG: -rpath [[MACOSX_SDK_STDLIB_PATH]]
 // IOS13-MACABI-DAG: -platform_version "mac catalyst" 13.0.0 0.0.0
 
+// Adjust iOS versions < 13.0 to 13.0 for the linker's sake.
+
+// RUN: %swiftc_driver -driver-print-jobs -target x86_64-apple-ios12.0-macabi -sdk %S/../Inputs/clang-importer-sdk %s | %FileCheck -check-prefix=IOS12-MACABI %s
+// IOS12-MACABI: bin/swift
+// IOS12-MACABI: -target x86_64-apple-ios12.0-macabi
+
+// IOS12-MACABI: bin/ld
+// IOS12-MACABI-DAG: -L [[MACCATALYST_STDLIB_PATH:[^ ]+/lib/swift/maccatalyst]]
+// IOS12-MACABI-DAG: -L [[MACOSX_STDLIB_PATH:[^ ]+/lib/swift/macosx]]
+// IOS12-MACABI-DAG: -L [[MACCATALYST_SDK_STDLIB_PATH:[^ ]+/clang-importer-sdk/System/iOSSupport/usr/lib/swift]]
+// IOS12-MACABI-DAG: -L [[MACOSX_SDK_STDLIB_PATH:[^ ]+/clang-importer-sdk/usr/lib/swift]]
+// IOS12-MACABI-DAG: -rpath [[MACCATALYST_STDLIB_PATH]]
+// IOS12-MACABI-DAG: -rpath [[MACOSX_STDLIB_PATH]]
+// IOS12-MACABI-DAG: -rpath [[MACCATALYST_SDK_STDLIB_PATH]]
+// IOS12-MACABI-DAG: -rpath [[MACOSX_SDK_STDLIB_PATH]]
+// IOS12-MACABI-DAG: -platform_version "mac catalyst" 13.0.0 0.0.0
 
 // Test using target-variant to build zippered outputs