[cxx-interop] Adding std.string initializer for UnsafePointer<CChar>?

stdlib/public/Cxx/std/String.swift

@@ -25,6 +25,20 @@ extension std.string {
       self.push_back(value_type(bitPattern: char))
     }
   }
+
+  public init(_ string: UnsafePointer<CChar>?) {
+    self.init()
+
+    guard let str = string else {
+      return
+    }
+
+    let len = strlen(str)
+    for i in 0..<len {
+      let char = UInt8(str[i])
+      self.push_back(value_type(bitPattern: char))
+    }
+  }
 }
 
 extension std.u16string {

test/Interop/Cxx/objc-correctness/init-String-with-NSString-utf8String.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -I %S/Inputs -cxx-interoperability-mode=swift-5.9 -emit-ir %s -Xcc -fignore-exceptions | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+import Foundation
+import CxxStdlib
+
+// CHECK: @"\01L_selector(UTF8String)"
+// CHECK: @objc_msgSend
+// CHECK: call swiftcc void @"$sSo3stdO3__1O0067basic_stringInt8char_traitsInt8allocatorInt8_FABErpaBGcqaGHerapGgqaV9CxxStdlibEyAFSPys4Int8VGSgcfC"
+
+let ObjCStr: NSString = "hello"
+let CxxStr = std.string(ObjCStr.utf8String) // Should not crash here

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

@@ -236,4 +236,11 @@ StdStringOverlayTestSuite.test("std::u16string as Swift.CustomStringConvertible"
   expectEqual(cxx3.description, "a�c")
 }
 
+StdStringOverlayTestSuite.test("std::string from C string") {
+  let str = "abc".withCString { ptr in
+    std.string(ptr)
+  }
+  expectEqual(str, std.string("abc"))
+}
+
 runAllTests()