Allow clients of the AST reader to specify what kinds of AST load
failures they know how to tolerate, e.g., out-of-date input files or
configuration/version mismatches. Suppress the corresponding
diagnostics if the client can handle it.

No clients actually use this functionality, yet.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166449 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed