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

Currently without an initializer for the unsafe char pointer type swiftc
hits an assert around not being able to handle conversions of unsafe
pointers with Any type. This patch adds the ability to convert to a
std::string.

This is to address issue #61218

Author: plotfi

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