[5.9] Fix a precondition failure if a generic parameter clause had an additional closing right angle

[ahoppen]

• Explanation: If a generic argument clause has an additional closing >, we performed a misparse, causing a precondition failure because we were consuming >> as a right angle, which doesn’t match the expected text.
• Scope: Parsing of generic argument argument clauses
• Risk: Very low. The old code was obviously wrong
• Testing: Added regression test
• Issue: rdar://108624264
• Reviewer: @bnbarham on Fix a precondition failure if a generic parameter clause had an additional closing right angle #1595

[ahoppen]

@swift-ci Please test