Add LTO support for WindowsToolchain

Author: stevapple

Sources/SwiftDriver/Jobs/WindowsToolchain+LinkerSupport.swift

@@ -117,6 +117,8 @@ extension WindowsToolchain {
           return .responseFilePath(input.file)
         } else if input.type == .object {
           return .path(input.file)
+        } else if lto != nil && input.type == .llvmBitcode {
+          return .path(input.file)
         } else {
           return nil
         }
@@ -162,6 +164,15 @@ extension WindowsToolchain {
         }
       }
 
+      if let lto = lto {
+        switch lto {
+        case .llvmFull:
+          commandLine.appendFlag("-flto=full")
+        case .llvmThin:
+          commandLine.appendFlag("-flto=thin")
+        }
+      }
+
       // Run clang++ in verbose mode if "-v" is set
       try commandLine.appendLast(.v, from: &parsedOptions)
 

Sources/SwiftDriver/Toolchains/WindowsToolchain.swift

@@ -137,7 +137,7 @@ extension WindowsToolchain {
       case .illegalCrtName(let argument):
         return "\(argument) is not a valid C Runtime for Windows"
       case .sdkNotFound:
-        return "swift development on Windows always requires SDK of the target platform"
+        return "swift development on Windows always requires the SDK of target platform"
       }
     }
   }