[Definite Initialization] Fix a bug that made the DI wrongly think that an unused access to "self" in a delegating initializer is a use of the form: type(of:self)

[ravikandhadai]

rdar://51198592

(cherry-picked from master ff3d081 PR: #25403)

[ravikandhadai]

@swift-ci Please smoke test

[ravikandhadai]

@swift-ci Please test Source Compatibility

[ravikandhadai]

@swift-ci Please Test Source Compatibility

[ravikandhadai]

@swift-ci please test

[ravikandhadai]

@swift-ci Please nominate

Explanation:
Swift compiler guarantees that memory locations are initialized before their use. This is enforced by a compiler pass: Definition Initialization (DI). In the case of convenience initializers of classes, this pass enforces that the 'self' reference is never used in any construct other than 'type(of: self)' before being initialized by a call to a designated initializer. The bug fixed by this patch causes DI to erroneously think that the convenience initializer has a 'type(of: self)' expression when there is none, which eventually results in a compiler crash.

Scope: The compiler crash fixed by this patch occurs frequently. It is consistently in the top 20 crashes of SourceKit.

Radar: rdar://problem/51198592

Risk:
This patch changes the behavior of the compiler from crashing to producing a compiler error. It affects only code containing convenience initializers that use 'self' before it is initialized. Any bugs in the fix could cause miscompiles and/or produce spurious compiler errors in such code.

Testing:
This patch is tested with Swift CI tests and with source compatibility tests. This patch was also merged with the open-source master branch and has been tested by the test bots without any issues.

[ravikandhadai]

Thanks Devin. This fix is also merged with 5.1-branch: #25499