[SR-7249] Compiler crash for redeclaration in extension of generic type

[hamishknight]

Previous ID	SR-7249
Radar	None
Original Reporter	@hamishknight
Type	Bug
Status	Closed
Resolution	Done

Environment

Swift version 4.2-dev (LLVM cef3fe6d75, Clang bdc970d89c, Swift 5b60ace)
Target: x86_64-apple-darwin17.4.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompilerCrash
Assignee	@hamishknight
Priority	Medium

md5: f79c841e907eb59984a6d43f8e0e7eee

Issue Description:

The following code crashes the compiler:

struct S<T> {
  var x: Int { return 0 }
}

extension S {
  var x: Int { return 0 }
}

It triggers an assertion in SILGen, but really Sema should catch this with an invalid redeclaration error.

[hamishknight]

#15412