[cxx-interop] Make String.init(std.string) unlabeled

`Swift.String` can be initialized from any other type with an unlabeled initializer, which is either going to use the `CustomStringConvertible` conformance, or reflection. We would like clients to use the most suitable initializer, which is the one that takes `std.string` as a parameter. For instance, that allows us to attach a doc comment to the initializer.

This change makes the initializer unlabeled to make sure it is chosed by overload resolution when a client invokes `String(myCxxString)`.

Author: egorzhdan

benchmark/cxx-source/CxxStringConversion.swift

@@ -54,7 +54,7 @@ public func run_swiftToCxx(_ n: Int) {
 public func run_cxxToSwift(_ n: Int) {
   let str = cxxString!
   for _ in 0..<n {
-    let x = String(cxxString: str)
+    let x = String(str)
     blackHole(x)
   }
 }

stdlib/public/Cxx/std/String.swift

@@ -31,13 +31,13 @@ extension std.string: ExpressibleByStringLiteral {
 
 extension std.string: CustomDebugStringConvertible {
   public var debugDescription: String {
-    return "std.string(\(String(cxxString: self)))"
+    return "std.string(\(String(self)))"
   }
 }
 
 extension std.string: CustomStringConvertible {
   public var description: String {
-    return String(cxxString: self)
+    return String(self)
   }
 }
 
@@ -49,7 +49,7 @@ extension String {
   /// (`"\u{FFFD}"`).
   ///
   /// - Complexity: O(*n*), where *n* is the number of bytes in the C++ string.
-  public init(cxxString: std.string) {
+  public init(_ cxxString: std.string) {
     let buffer = UnsafeBufferPointer<CChar>(
       start: cxxString.__c_strUnsafe(),
       count: cxxString.size())

test/Interop/Cxx/stdlib/overlay/std-string-overlay.swift

@@ -10,11 +10,11 @@ var StdStringOverlayTestSuite = TestSuite("std::string overlay")
 
 StdStringOverlayTestSuite.test("std::string <=> Swift.String") {
   let cxx1 = std.string()
-  let swift1 = String(cxxString: cxx1)
+  let swift1 = String(cxx1)
   expectEqual(swift1, "")
 
   let cxx2 = std.string("something123")
-  let swift2 = String(cxxString: cxx2)
+  let swift2 = String(cxx2)
   expectEqual(swift2, "something123")
 
   let cxx3: std.string = "literal"
@@ -23,25 +23,25 @@ StdStringOverlayTestSuite.test("std::string <=> Swift.String") {
   // Non-ASCII characters are represented by more than one CChar.
   let cxx4: std.string = "тест"
   expectEqual(cxx4.size(), 8)
-  let swift4 = String(cxxString: cxx4)
+  let swift4 = String(cxx4)
   expectEqual(swift4, "тест")
 
   let cxx5: std.string = "emoji_🤖"
   expectEqual(cxx5.size(), 10)
-  let swift5 = String(cxxString: cxx5)
+  let swift5 = String(cxx5)
   expectEqual(swift5, "emoji_🤖")
 
   let cxx6 = std.string("xyz\0abc")
   expectEqual(cxx6.size(), 7)
-  let swift6 = String(cxxString: cxx6)
+  let swift6 = String(cxx6)
   expectEqual(swift6, "xyz\0abc")
 
   var cxx7 = std.string()
   let bytes: [UInt8] = [0xE1, 0xC1, 0xAC]
   for byte in bytes {
     cxx7.push_back(CChar(bitPattern: byte))
   }
-  let swift7 = String(cxxString: cxx7)
+  let swift7 = String(cxx7)
   expectEqual(swift7, "���")
 }