Keep track of modules that have failed to build. If we encounter an
import of that module elsewhere, don't try to build the module again:
it won't work, and the experience is quite dreadful. We track this
information somewhat globally, shared among all of the related
CompilerInvocations used to build modules on-the-fly, so that a
particular Clang instance will only try to build a given module once.

Fixes <rdar://problem/12552849>.




git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168961 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed