[6.0] SILGen: ignore unreachable var decls

[eeckstein]

• Explanation: Fixes a crash in case a lazy var is declared after a return statement. SILGen didn't handle variable declarations correctly if they appear in unreachable code, e.g. after a return statement
• Scope: Only affects code where some kind of variable declarations (e.g. lazy variables) appear after a return (or similar) statement.
• Risk: low. The change is simple and only affects (unintentionally) unreachable code.
• Testing: Tested by a test case
• Issue: rdar://129694376, SILGen assertion failure when compiling unreachable lazy local variable #73736
• Reviewer: @jckarter
• Main branch PR: SILGen: ignore unreachable var decls #74256

[eeckstein]

@swift-ci test