[SR-9050] error: cannot load underlying module for 'Dispatch'

Issue Description:

Log:

$ docker run --privileged -it --rm norionomura/swift:20181019a swift
Welcome to Swift version 4.2-dev (LLVM c4a0883115, Clang 0a99881462, Swift a640fe7dd6).
Type :help for assistance.
  1> import Dispatch
warning: Swift error in fallback scratch context: error: repl.swift:1:8: error: cannot load underlying module for 'Dispatch'
import Dispatch
       ^



note: This error message is displayed only once. If the error displayed above is due to conflicting search paths to Clang modules in different images of the debugged executable, this can slow down debugging of Swift code significantly, since a fresh Swift context has to be created every time a conflict is encountered.

expression failed to parse, unknown error
  1>  

On swift-DEVELOPMENT-SNAPSHOT-2018-10-19-a, it seems /usr/lib/swift/dispatch/module.modulemap changed to symbolic link against /home/buildnode/jenkins/workspace/oss-swift-package-linux-ubuntu-16_04/swift-corelibs-libdispatch/dispatch/generic/module.modulemap that does not exists.

 ls -la /usr/lib/swift/dispatch/
total 216
drwxr-xr-x  2 root root  4096 Oct 19 16:44 .
drwxr-xr-x 11 root root  4096 Oct 19 16:44 ..
-rw-r--r--  1 root root  8155 Oct 19 15:42 base.h
-rw-r--r--  1 root root 17651 Oct 19 15:42 block.h
-rw-r--r--  1 root root 10865 Oct 19 15:42 data.h
-rw-r--r--  1 root root  1994 Oct 19 15:42 dispatch.h
-rw-r--r--  1 root root  8412 Oct 19 15:42 group.h
-rw-r--r--  1 root root  4894 Oct 19 15:42 introspection.h
-rw-r--r--  1 root root 25802 Oct 19 15:42 io.h
lrwxrwxrwx  1 root root   131 Oct 19 16:46 module.modulemap -> /home/buildnode/jenkins/workspace/oss-swift-package-linux-ubuntu-16_04/swift-corelibs-libdispatch/dispatch/generic/module.modulemap
-rw-r--r--  1 root root 17247 Oct 19 15:42 object.h
-rw-r--r--  1 root root  3633 Oct 19 15:42 once.h
-rw-r--r--  1 root root 50416 Oct 19 15:42 queue.h
-rw-r--r--  1 root root  3321 Oct 19 15:42 semaphore.h
-rw-r--r--  1 root root 27456 Oct 19 15:42 source.h
-rw-r--r--  1 root root  3020 Oct 19 15:42 time.h

Reproduce the failure with the documented Docker command and inspect /usr/lib/swift/dispatch/module.modulemap, especially its broken symlink target. Compare the installed module map with the DEVELOPMENT-2018-11-21-a and 2018-10-19-a snapshots, then verify that importing Dispatch succeeds and the installed link points to an existing file.