[CSGen] Adjust locators for sub-patterns of parens

In cases like `.<name>(_)`, the `_` sub-pattern should not assume
locator of the parent paren, just like it doesn't in case of tuple
i.e. `.<name>(_, ...)` where each element gets a `PatternMatch(<elt>)`
locator.

Author: xedin

lib/Sema/CSGen.cpp

@@ -2265,9 +2265,11 @@ namespace {
               ->getUnderlyingType();
         }
 
-        auto underlyingType =
-            getTypeForPattern(paren->getSubPattern(), locator,
-                              externalPatternType, bindPatternVarsOneWay);
+        auto *subPattern = paren->getSubPattern();
+        auto underlyingType = getTypeForPattern(
+            subPattern,
+            locator.withPathElement(LocatorPathElt::PatternMatch(subPattern)),
+            externalPatternType, bindPatternVarsOneWay);
 
         if (!underlyingType)
           return Type();
@@ -2694,7 +2696,9 @@ namespace {
           // and we're matching the type of that subpattern to the parameter
           // types.
           Type subPatternType = getTypeForPattern(
-              subPattern, locator, Type(), bindPatternVarsOneWay);
+              subPattern,
+              locator.withPathElement(LocatorPathElt::PatternMatch(subPattern)),
+              Type(), bindPatternVarsOneWay);
 
           if (!subPatternType)
             return Type();

test/Constraints/patterns.swift

@@ -486,6 +486,7 @@ func rdar63510989() {
   }
 
   func test(e: E) {
+    if case .single(_) = e {} // Ok
     if case .single(_ as Value) = e {} // Ok
     if case .single(let v as Value) = e {} // Ok
     // expected-warning@-1 {{immutable value 'v' was never used; consider replacing with '_' or removing it}}