[Modules] Support #import when entering files with modules

Textual headers and builtins that are #import'd from different
modules should get re-entered when these modules are independent
from each o

[Modules] Support #import when entering files with modules

Textual headers and builtins that are #import'd from different
modules should get re-entered when these modules are independent
from each other.

Differential Revision: https://reviews.llvm.org/D26267

rdar://problem/25881934

llvm-svn: 291644

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