Fix #4732: Roots the desugared call to StringContext.

* Make desugared `s""` call `_root_.scala.StringContext` instead of just
  `StringContext`
* Add a unit test

Author: lloydmeta

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1109,8 +1109,13 @@ object desugar {
           case Block(Nil, expr) => expr // important for interpolated string as patterns, see i1773.scala
           case t => t
         }
-
-        Apply(Select(Apply(Ident(nme.StringContext), strs), id), elems)
+        // This is a deliberate departure from scalac, where StringContext is not rooted (See #4732)
+        val rootedStringContext = {
+          val rootPkg = Ident(nme.ROOTPKG)
+          val scalaPkg = Select(rootPkg, nme.scala_)
+          Select(scalaPkg, nme.StringContext)
+        }
+        Apply(Select(Apply(rootedStringContext, strs), id), elems)
       case InfixOp(l, op, r) =>
         if (ctx.mode is Mode.Type)
           AppliedTypeTree(op, l :: r :: Nil) // op[l, r]

tests/run/rooted_stringcontext.check

@@ -0,0 +1 @@
+Answer: 42

tests/run/rooted_stringcontext.scala

@@ -0,0 +1,10 @@
+object StringContext {
+  val i = 42
+  val s = s"Answer: $i"
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(StringContext.s)
+  }
+}