[lldb] Use function in TestSwiftEmbeddedExpression so its not deleted

The compiler will not keep functions only for the debugger when
compiling in embedded Swift mode. Change TestSwiftEmbeddedExpression so
the function being called is not optimized out.

Author: augusto2112

lldb/test/API/lang/swift/embedded/expr/main.swift

@@ -1,7 +1,7 @@
-struct A {
+public struct A {
   var field = 4
 
-  func foo() -> Int {
+  public func foo() -> Int {
     return field * field
   }
 }