Basic: Change isSwiftVersionAtLeast() to take optional minor version number

I want to be able to do an isSwiftVersionAtLeast(4, 2) check.

Author: slavapestov

include/swift/AST/ASTContext.h

@@ -880,8 +880,8 @@ class ASTContext final {
   /// This is usually the check you want; for example, when introducing
   /// a new language feature which is only visible in Swift 5, you would
   /// check for isSwiftVersionAtLeast(5).
-  bool isSwiftVersionAtLeast(unsigned major) const {
-    return LangOpts.isSwiftVersionAtLeast(major);
+  bool isSwiftVersionAtLeast(unsigned major, unsigned minor = 0) const {
+    return LangOpts.isSwiftVersionAtLeast(major, minor);
   }
 
 private:

include/swift/Basic/LangOptions.h

@@ -342,8 +342,8 @@ namespace swift {
     /// This is usually the check you want; for example, when introducing
     /// a new language feature which is only visible in Swift 5, you would
     /// check for isSwiftVersionAtLeast(5).
-    bool isSwiftVersionAtLeast(unsigned major) const {
-      return EffectiveLanguageVersion.isVersionAtLeast(major);
+    bool isSwiftVersionAtLeast(unsigned major, unsigned minor = 0) const {
+      return EffectiveLanguageVersion.isVersionAtLeast(major, minor);
     }
 
     /// Returns true if the given platform condition argument represents

include/swift/Basic/Version.h

@@ -112,8 +112,17 @@ class Version {
 
   /// Whether this version is greater than or equal to the given major version
   /// number.
-  bool isVersionAtLeast(unsigned major) const {
-    return !empty() && Components[0] >= major;
+  bool isVersionAtLeast(unsigned major, unsigned minor = 0) const {
+    switch (size()) {
+    case 0:
+      return false;
+    case 1:
+      return ((Components[0] == major && 0 == minor) ||
+              (Components[0] > major));
+    default:
+      return ((Components[0] == major && Components[1] >= minor) ||
+              (Components[0] > major));
+    }
   }
 
   /// Return this Version struct with minor and sub-minor components stripped