It is possible for the SIL optimizers, IRGen, etc. to request information
from the AST that only the type checker can provide, but the type checker
is typically torn down after the “type checking” phase. This can lead to
various crashes late in the compilation cycle.
Keep the type checker instance around as long as the ASTContext is alive
or until someone asks for it to be destroyed.
Fixes SR-285 / rdar://problem/23677338.
Take away the type checker constructor that allows one to provide a
diagnostic engine different from the one associated with the ASTContext. It
doesn’t actually work to suppress diagnostics. Switch all clients over to
the constructor that takes only an ASTContext.
Introduce the DiagnosticSuppression RAII class so clients that want to
suppress diagnostics can suppress *all* diagnostics. Use it where we
were previously suppressing diagnostics.
Replace `NameOfType foo = dyn_cast<NameOfType>(bar)` with DRY version `auto foo = dyn_cast<NameOfType>(bar)`.
The DRY auto version is by far the dominant form already used in the repo, so this PR merely brings the exceptional cases (redundant repetition form) in line with the dominant form (auto form).
See the [C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es11-use-auto-to-avoid-redundant-repetition-of-type-names) for a general discussion on why to use `auto` to avoid redundant repetition of type names.
Based off the PlaygroundTransform, this new ASTWalker leaves calls to __builtin_pc_before and __builtin_pc_after before and after a user would expect a program counter to enter a range of source code.
