Don't create a Module SwiftASTContext when the stdlib is missing

[vedantk]

A Module SwiftASTContext that cannot access the stdlib module will crash
when performing simple actions, like looking up a type.

This patch tries to make the Target/Module overrides of CreateInstance
look more like each other, w.r.t error handling in particular.

rdar://57695158, rdar://64828733
(cherry picked from commit 21acf0e)

[vedantk]

PR for master: #1391. (master-next has the stylistic changes Adrian requested.)

[fredriss]

@swift-ci test

[fredriss]

Do we know what can lead to this kind of situation in the first place?

[vedantk]

Do we know what can lead to this kind of situation in the first place?

There's two situations I know of. One is the case where the program doesn't link in the stdlib. The other is when there's a version mismatch between the debugger and the compiler, like: Cannot load Swift type information; AST validation error in "[redacted]": The module file format is too new to be used by this version of the debugger. Cannot create Swift scratch context (couldn't load the Swift stdlib). In the latter case at least (not sure about the former), when we fail to create a scratch AST context, we try to create a fallback Module AST context, and this can accidentally succeed even if the stdlib is missing. A missing stdlib is an error when constructing the scratch context.

[adrian-prantl]

Thanks!