stdlib: changes for the StringOptimization

To be able to constant fold string interpolation, the right semantic attributes must be in place.
Also, the interpolation's write function must be inlinable.
For the _typeName constant folding, a semantic attribute is required.

Author: eeckstein

stdlib/public/core/Misc.swift

@@ -71,6 +71,7 @@ public func _getTypeName(_ type: Any.Type, qualified: Bool)
   -> (UnsafePointer<UInt8>, Int)
 
 /// Returns the demangled qualified name of a metatype.
+@_semantics("typeName")
 public // @testable
 func _typeName(_ type: Any.Type, qualified: Bool = true) -> String {
   let (stringPtr, count) = _getTypeName(type, qualified: qualified)

stdlib/public/core/OutputStream.swift

@@ -557,6 +557,7 @@ extension String: TextOutputStreamable {
   /// Writes the string into the given output stream.
   ///
   /// - Parameter target: An output stream.
+  @inlinable
   public func write<Target: TextOutputStream>(to target: inout Target) {
     target.write(self)
   }

stdlib/public/core/String.swift

@@ -359,6 +359,18 @@ public struct String {
     _invariantCheck()
   }
 
+  // This is intentionally a static function and not an initializer, because
+  // an initializer would conflict with the Int-parsing initializer, when used
+  // as function name, e.g.
+  //   [1, 2, 3].map(String.init)
+  @_alwaysEmitIntoClient
+  @_semantics("string.init_empty_with_capacity")
+  @_semantics("inline_late")
+  @inlinable
+  internal static func _createEmpty(withInitialCapacity: Int) -> String {
+    return String(_StringGuts(_initialCapacity: withInitialCapacity))
+  }
+
   /// Creates an empty string.
   ///
   /// Using this initializer is equivalent to initializing a string with an

stdlib/public/core/StringInterpolation.swift

@@ -75,7 +75,7 @@ public struct DefaultStringInterpolation: StringInterpolationProtocol {
     let capacityPerInterpolation = 2
     let initialCapacity = literalCapacity +
       interpolationCount * capacityPerInterpolation
-    _storage = String(_StringGuts(_initialCapacity: initialCapacity))
+    _storage = String._createEmpty(withInitialCapacity: initialCapacity)
   }
 
   /// Appends a literal segment of a string interpolation.