[cxx-interop][SwiftCompilerSources] Fix conversion between std::string and Swift.String

This fixes a dangling pointer issue when creating a `Swift.String` from `std::string`.

Also fixes a warning:
```
warning: variable 's' was never mutated; consider changing to 'let' constant
    var s = SILBasicBlock_debugDescription(bridged)
    ~~~ ^
    let
```

rdar://92963081
rdar://93053488

SwiftCompilerSources/Sources/Basic/Utils.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 @_exported import BasicBridging
+import std
 
 //===----------------------------------------------------------------------===//
 //                              StringRef
@@ -60,6 +61,13 @@ extension String {
       return c(BridgedStringRef(data: buffer.baseAddress, length: buffer.count))
     }
   }
+
+  /// Underscored to avoid name collision with the std overlay.
+  /// To be replaced with an overlay call once the CI uses SDKs built with Swift 5.8.
+  public init(_cxxString s: std.string) {
+    self.init(cString: s.c_str())
+    withExtendedLifetime(s) {}
+  }
 }
 
 extension Array {