[gardening] Disambiguate the meaning of Expr::getEndLoc() in the doc comment

"end of the expression" is very confusing. getEndLoc() actually returns
the start location of the last token of the expression.

Author: rintaro

include/swift/AST/Expr.h

@@ -395,7 +395,7 @@ class alignas(8) Expr {
   /// getStartLoc - Return the location of the start of the expression.
   SourceLoc getStartLoc() const;
 
-  /// \brief Retrieve the location of the end of the expression.
+  /// \brief Retrieve the location of the last token of the expression.
   SourceLoc getEndLoc() const;
 
   /// getLoc - Return the caret location of this expression.