clang/Modules: Split loop in ReadAST between failable and not

Split a loop in ReadAST that visits the just-loaded module chain,
between an initial loop that reads further from the ASTs (and can fail)
and a second loop that does some preloading (and cannot fail).  This
makes it less likely for a reading failure to affect the AST.

This is not fixing a known bug and the behaviour change may not be
observable, it's just part of an audit to look at all of the error
handling in the ASTReader.

https://reviews.llvm.org/D70056