[6.0][SourceLocationConverter] Performance improvement

[rintaro]

Cherry-pick https://github.com/apple/swift-syntax/pull/2663 into release/6.0

• Explanation: Improve the performance of SourceLocationConverter initialization, at the cost of a bit readability.
• Scope: Source location calculation
• Risk: Low. No API changes, no use of risky experimental features.
• Testing: Passes current test suite, also added a few test cases to verify
• Issues: rdar://128618516
• Reviewer: Hamish Knight (@hamishknight) Alex Hoppen (@ahoppen)

[rintaro]

@swift-ci Please test