Don't synthesize an ImportDecl for a module named in -fmodule-implementation-of

When building a PCH with modules enabled this import would assert in the
ASTWriter and (if assertions were disabled) sometimes crash the compiler
that loaded the resulting PCH when trying to lookup the submodule ID.

rdar://problem/24137448

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259859 91177308-0d34-0410-b5e6-96231b3b80d8