[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/SIL/Function.swift

@@ -21,8 +21,7 @@ final public class Function : CustomStringConvertible, HasShortDescription {
   }
 
   final public var description: String {
-    var s = SILFunction_debugDescription(bridged)
-    return String(cString: s.c_str())
+    String(_cxxString: SILFunction_debugDescription(bridged))
   }
 
   public var shortDescription: String { name.string }