[OpenACC][NFC] Fix EndLoc behavior of optional clause params

It was discovered while writing the 'wait' clause ast tests that the
'endloc' wasn't set correctly when there was no arguments.  This patch
ensures we set it right, and adds an assert to prevent us from messing
it up in the future.

Author: erichkeane

clang/include/clang/AST/OpenACCClause.h

@@ -26,7 +26,10 @@ class OpenACCClause {
 protected:
   OpenACCClause(OpenACCClauseKind K, SourceLocation BeginLoc,
                 SourceLocation EndLoc)
-      : Kind(K), Location(BeginLoc, EndLoc) {}
+      : Kind(K), Location(BeginLoc, EndLoc) {
+    assert(!BeginLoc.isInvalid() && !EndLoc.isInvalid() &&
+           "Begin and end location must be valid for OpenACCClause");
+      }
 
 public:
   OpenACCClauseKind getClauseKind() const { return Kind; }

clang/lib/Parse/ParseOpenACC.cpp

@@ -1112,6 +1112,10 @@ Parser::OpenACCClauseParseResult Parser::ParseOpenACCClauseParams(
       ParsedClause.setEndLoc(getCurToken().getLocation());
       if (Parens.consumeClose())
         return OpenACCCannotContinue();
+    } else {
+      // If we have optional parens, make sure we set the end-location to the
+      // clause, as we are a 'single token' clause.
+      ParsedClause.setEndLoc(ClauseLoc);
     }
   }
   return OpenACCSuccess(