IndexSet's find family of methods should have optional returns

[parkera]

What's in this pull request?

NSIndexSet returns NSNotFound for its find family of methods, but struct IndexSet should use an optional return value instead.

Resolved bug number

rdar://problem/27300571

---

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Smoke Testing

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All supported platforms	@swift-ci Please smoke test and merge
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Linux platform	@swift-ci Please smoke test Linux platform

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A smoke test on Linux does the following:

1. Builds the compiler incrementally.
2. Builds the standard library incrementally.
3. lldb is built incrementally.
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5. lldb is tested.

Validation Testing

Platform	Comment
All supported platforms	@swift-ci Please test
All supported platforms	@swift-ci Please test and merge
OS X platform	@swift-ci Please test OS X platform
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Lint Testing

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[parkera]

@swift-ci Please test and merge

[parkera]

ERROR: [GitHub Commit Status Setter] - {"message":"No commit found for SHA: ee7844736368987235a207727c495bad23e62d4a","documentation_url":"https://developer.github.com/v3/repos/statuses/"}, setting build result to FAILURE
Build step 'Set build status on GitHub commit [deprecated]' changed build result to FAILURE
[BFA] Scanning build for known causes...
.[BFA] No failure causes found
[BFA] Done. 1s
Finished: FAILURE

[parkera]

@swift-ci Please test and merge

[parkera]

@swift-ci Please test and merge

[parkera]

Can't merge to master, but the rest of the build succeeded.

[parkera]

@swift-ci Please test and merge

[parkera]

@swift-ci please smoke test

[parkera]

[xUnit] [INFO] - [JUnit] - 1 test report file(s) were found with the pattern 'buildbot_linux/swift-linux-x86_64/swift-test-results/x86_64-unknown-linux-gnu/**.xml' relative to '/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test' for the testing framework 'JUnit'.
[xUnit] [ERROR] - Test reports were found but not all of them are new. Did all the tests run?
  * /home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/buildbot_linux/swift-linux-x86_64/swift-test-results/x86_64-unknown-linux-gnu/lit-tests.xml is 15 hr old

[parkera]

@swift-ci please smoke test linux

[parkera]

@swift-ci please smoke test linux

[parkera]

This passed on macOS and is not part of Linux (overlay code), so I'm merging this.