swift234

Start building conformances when requested in the AST.

Previously, we would require the type checker to be able to build a
conformance, which meant we would actually have to lie in the AST
about having a conformance (or crash; we did the form). Now, we can
form the conformance in the AST and it will be checked in the type
checker when needed. The intent here is to push conformance creation
into the conformance lookup table.

To get here, we had to stop relying on the broken, awful,
ASTContext-wide conformance "cache". A proper cache can come back once
the model is sorted out.

Swift SVN r26250