[Modules] Make ReadModuleMapFileBlock errors reliable

This prevents a crash when an error should be emitted instead.

During implicit module builds, there are cases where ReadASTCore is called with
ImportedBy set to nullptr, which breaks expectations in ReadModuleMapFileBlock,
leading to crashes.

Fix this by improving ReadModuleMapFileBlock to handle ImportedBy correctly.
This only happens non deterministically in the wild, when the underlying file
system changes while concurrent compiler invocations use implicit modules,
forcing rebuilds which see an inconsistent filesystem state. That said, there's
no much to do w.r.t. writing tests here.

rdar://problem/48828801

llvm-svn: 370422

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