Propagate the DEFAULTINIT flag to the underlying field

Fields initialized with `_` are excluded from initialization
checks under `-Xcheckinit`.

Since 5f86b1d, the compiler checks that flag on the field symbol
instead of the getter. Unfortunately the flag was not actually copied
to the field symbol, causing the init check to be added.

Fixes scala/bug#10439, scala/bug#10437.

Author: lrytz

src/reflect/scala/reflect/internal/Flags.scala

@@ -268,7 +268,7 @@ class Flags extends ModifierFlags {
    *  PRIVATE, LOCAL.
    */
   final val FieldFlags =
-    MUTABLE | CASEACCESSOR | PARAMACCESSOR | STATIC | FINAL | PRESUPER | LAZY
+    MUTABLE | CASEACCESSOR | PARAMACCESSOR | STATIC | FINAL | PRESUPER | LAZY | DEFAULTINIT
 
   /** Masks for getters and setters, where the flags are derived from those
    *  on the field's modifiers.  Both getters and setters get the ACCESSOR flag.

test/files/run/t10439.flags

@@ -0,0 +1 @@
+-Xcheckinit

test/files/run/t10439.scala

@@ -0,0 +1,14 @@
+object Test {
+  private var s: String = _
+
+  def getS: String = {
+    if (s == null) {
+      s = ""
+    }
+    s
+  }
+
+  def main(args: Array[String]): Unit = {
+    assert(getS == "")
+  }
+}