[6.2] [Parse] Avoid parsing unsafe expression before binary or postfix op

[hamishknight]

6.2 cherry-pick of #81429

• Explanation: Avoids parsing an unsafe expression if followed by a binary or postfix operator.
• Scope: Affects parsing of unsafe.
• Issue: Compilation error: expected expression after 'unsafe' #81323, rdar://150751248
• Risk: Low, avoids parsing unsafe expression in cases that would be invalid, adjusts logic to more closely align with the swift-syntax parser logic.
• Testing: Added tests to test suite
• Reviewer: Hamish Knight

[hamishknight]

@swift-ci please test