Tweak Swift-in-OS check to account for linker limitations

The backwards-deployment install name trickery we're using doesn't
handle "patch" components in version numbers, so we still need to
provide an rpath even when deploying to macOS 10.14.4.

Author: jrose-apple

include/swift/Basic/Platform.h

@@ -45,9 +45,9 @@ namespace swift {
   /// Return true if the given triple represents any simulator.
   bool tripleIsAnySimulator(const llvm::Triple &triple);
 
-  /// Returns true if the given triple represents an OS that ships with ABI-stable
-  /// swift libraries (eg. in /usr/lib/swift).
-  bool tripleHasSwiftInTheOS(const llvm::Triple &triple);
+  /// Returns true if the given triple represents an OS that ships with
+  /// ABI-stable swift libraries (eg. in /usr/lib/swift).
+  bool tripleRequiresRPathForSwiftInOS(const llvm::Triple &triple);
 
   /// Returns the platform name for a given target triple.
   ///

lib/Basic/Platform.cpp

@@ -55,21 +55,20 @@ bool swift::tripleIsAnySimulator(const llvm::Triple &triple) {
 }
 
 
-bool swift::tripleHasSwiftInTheOS(const llvm::Triple &triple) {
-  unsigned major, minor, revision;
+bool swift::tripleRequiresRPathForSwiftInOS(const llvm::Triple &triple) {
   if (triple.isMacOSX()) {
-    triple.getMacOSXVersion(major, minor, revision);
-    return llvm::VersionTuple(major, minor, revision) >=
-      llvm::VersionTuple(10, 14, 4);
+    // macOS 10.14.4 contains a copy of Swift, but the linker will still use an
+    // rpath-based install name until 10.15.
+    return triple.isMacOSXVersionLT(10, 15);
+
   } else if (triple.isiOS()) {
-    triple.getiOSVersion(major, minor, revision);
-    return llvm::VersionTuple(major, minor, revision) >=
-      llvm::VersionTuple(12, 2);
+    return triple.isOSVersionLT(12, 2);
+
   } else if (triple.isWatchOS()) {
-    triple.getOSVersion(major, minor, revision);
-    return llvm::VersionTuple(major, minor, revision) >=
-      llvm::VersionTuple(5, 2);
+    return triple.isOSVersionLT(5, 2);
   }
+
+  // Other platforms don't have Swift installed as part of the OS by default.
   return false;
 }
 

lib/Driver/DarwinToolChains.cpp

@@ -350,7 +350,7 @@ toolchains::Darwin::addArgsToLinkStdlib(ArgStringList &Arguments,
       Arguments.push_back("-rpath");
       Arguments.push_back(context.Args.MakeArgString(RuntimeLibPath));
 
-    } else if (tripleHasSwiftInTheOS(getTriple()) ||
+    } else if (!tripleRequiresRPathForSwiftInOS(getTriple()) ||
                context.Args.hasArg(options::OPT_no_stdlib_rpath)) {
       // If targeting an OS with Swift in /usr/lib/swift, the LC_ID_DYLIB
       // install_name the stdlib will be an absolute path like