Make '( and '{ tokens that can start an expression

Fixes a bug in parsing quotes.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Tokens.scala

@@ -211,7 +211,7 @@ object Tokens extends TokensCommon {
     USCORE, NULL, THIS, SUPER, TRUE, FALSE, RETURN, XMLSTART)
 
   final val canStartExpressionTokens = atomicExprTokens | BitSet(
-    LBRACE, LPAREN, IF, DO, WHILE, FOR, NEW, TRY, THROW)
+    LBRACE, LPAREN, QBRACE, QPAREN, IF, DO, WHILE, FOR, NEW, TRY, THROW)
 
   final val canStartTypeTokens = literalTokens | identifierTokens | BitSet(
     THIS, SUPER, USCORE, LPAREN, AT)