Parse a possible trailing postfix expression suffix after a fold expression

Summary:
This patch allows the parsing of a postfix expression involving a fold expression, which is legal as a fold-expression is a primary-expression.

See also https://llvm.org/pr38282

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49848

llvm-svn: 338170