[SR-6] Improve the AST verifier

[jopamer]

Previous ID	SR-6
Radar	rdar://problem/17858620
Original Reporter	@jopamer
Type	Task
Status	Reopened
Resolution

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Task
Assignee	None
Priority	Medium

md5: c8813366828ca436433065a7b8e88402

Issue Description:

We get a lot of bugs that land in backend components because they crash in SILGen, optimization, or IRGen, but are really rooted in type-checker bugs. To help better screen these bugs, and to hopefully catch them sooner, we should put some effort into the AST verifier to make it check more invariants. Currently it misses a lot of basic things, like 'ParenExprs shouldn't change the type of the subexpression' or '*MemberRef should reference a member that's actually reachable from the base type'.