[5.2][cmake] Add support for exporting frameworks/libraries into cmake exp…

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…ort files.

I think this was just an oversight. The new cmake 3.16 seems to choke if we do
not add SourceKit to the exports file since there are dependencies upon it in
other swift libraries.

(cherry picked from commit 8d712af)

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@swift-ci test

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Build failed
Swift Test Linux Platform
Git Sha - 259afb2

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@gottesmm It's not clear how changing the sourcekit cmake broke this... I'll PR test again to see what happens...

3:59:40 + /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-swift-5.2-branch/cmake-linux-x86_64/bin/cmake --build /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-swift-5.2-branch/buildbot_linux/cmark-linux-x86_64 -- -j16 test
13:59:40 [0/1][  0%][0.000s] Running tests...
13:59:40 Test project /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-swift-5.2-branch/buildbot_linux/cmark-linux-x86_64
13:59:40     Start 1: api_test
13:59:40 1/9 Test #1: api_test .........................   Passed    0.00 sec
13:59:40     Start 2: html_normalization
13:59:40 2/9 Test #2: html_normalization ...............   Passed    0.53 sec
13:59:40     Start 3: spectest_library
13:59:43 3/9 Test #3: spectest_library .................   Passed    2.79 sec
13:59:43     Start 4: pathological_tests_library
13:59:57 4/9 Test #4: pathological_tests_library .......***Failed   13.58 sec
13:59:57     Start 5: roundtriptest_library
13:59:58 5/9 Test #5: roundtriptest_library ............   Passed    1.14 sec
13:59:58     Start 6: entity_library
13:59:59 6/9 Test #6: entity_library ...................   Passed    0.64 sec
13:59:59     Start 7: spectest_executable
14:01:08 7/9 Test #7: spectest_executable ..............   Passed   69.55 sec
14:01:08     Start 8: smartpuncttest_executable
14:01:12 8/9 Test #8: smartpuncttest_executable ........   Passed    3.84 sec
14:01:12     Start 9: regressiontest_executable
14:01:14 9/9 Test #9: regressiontest_executable ........   Passed    2.51 sec
14:01:15 
14:01:15 89% tests passed, 1 tests failed out of 9
14:01:15 
14:01:15 Total Test time (real) =  94.71 sec
14:01:15 
14:01:15 The following tests FAILED:
14:01:15 	  4 - pathological_tests_library (Failed)
14:01:15 Errors while running CTest
14:01:15 FAILED: CMakeFiles/test.util 

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@swift-ci test linux

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@gottesmm this needs CCC to be merged.

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@gottesmm I'm not the right person to writeup CCC for this. Feel from to create your own PR, or attach your CCC to this one (which has passed the tests).

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--- CCC for 5.2 merge ---

Original Author: Michael Gottesman

Explanation:

Swift's CMake communicates dependencies to downstream cmake projects via SwiftConfig.cmake. This is done by "exporting" the dependency into the config file. The exporting results in that file containing the declaration of an imported target. When the downstream cmake wants to use binaries provided by the imported target, it just includes the SwiftConfig.cmake file and gets all of the name/location it needs.
In older versions of cmake, we were able to get away with not actually exporting sourcekitd, but successfully linking. The new CMake seems to be significantly stricter and wants us to actually create a true export for sourcekitd. This commit adds that support. There should be no functional change here since we were definitely linking against sourcekitd before. (edited)

Scope: Most local development machines are running cmake 3.16. All these machines will fail to build the swift-5.2-branch because it did not pickup this cmake fix from master.

Risk: Zero

Testing: Local build and PR test

Reviewer: Ben Langmuir

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@swift-ci test