Fixes String iterpolator with \"

Fixes scala/bug#6476

SIP-11 specification says

```
interpolatedString ::= alphaid `"` {printableChar \ (`"` | `$`) | escape} `"`
```

My reading of this is that the scanner should support the character sequence `\"`, but pass them as is. The Standard (s) interpolator would take care of the evaluation of `\"` as `"`.
This would fix the long standing issue of people trying to write `s"a=\"$a\""` and running into mysterious `';' expected but string literal found.`.

This change should be relatively safe since neither `s"\"foo\""` nor `s"foo\"` previously compiled.

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -907,6 +907,20 @@ trait Scanners extends ScannersCommon {
           setStrVal()
           token = STRINGLIT
         }
+      } else if (ch == '\\') {
+        if (multiLine) {
+          putChar(ch)
+          nextRawChar()
+          getStringPart(multiLine)
+        } else {
+          putChar(ch)
+          nextRawChar()
+          if (ch == '"') {
+            putChar(ch)
+            nextRawChar()
+          }
+          getStringPart(multiLine)
+        }
       } else if (ch == '$') {
         nextRawChar()
         if (ch == '$') {

test/files/run/string-interpolator.check

@@ -0,0 +1 @@
+"Hello", Alice

test/files/run/string-interpolator.scala

@@ -0,0 +1,6 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val person = "Alice"
+    println(s"\"Hello\", $person")
+  }
+}