Parse matching pattern specific productions at top level.

Parse 'var', '_', and 'is' pattern forms at the top level of a matching pattern context. Keep track of VarPatternDepth in the parser state and raise an error if 'var' appears in a 'var'.

Swift SVN r5839

test/Parse/matching_patterns.swift

@@ -0,0 +1,30 @@
+// RUN: %swift -dump-parse -verify %s
+
+var x:Int
+
+func square(x:Int) -> Int { return x*x }
+
+struct A<B> {
+  struct C<D> { }
+}
+
+switch x {
+// Expressions as patterns.
+case 0:
+case 1 + 2:
+case square(9):
+
+// 'var' pattern.
+case var a:
+case var var a: // expected-error{{'var' cannot appear nested inside another 'var' pattern}}
+
+// 'Any' pattern.
+case _:
+
+// 'is' pattern.
+case is A<B>:
+case is A<B>.C<D>:
+case is (Int, Int):
+case is (a:Int, b:Int):
+
+}