[6.0][Caching] Handle emit module job correctly for swift caching #1603
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Explanation: Fix cache key mis-match between swift-frontend and swift-driver that can cause caching inconsistency when only emit module job is scheduled by swift-driver.
Scope: Fixes issue that build system can wrongly think cache miss when it is actually cache hit, or misidentify certain job as not cacheable and always run the compilation.
Issue: rdar://127768967
Original PR: #1602
Testing: unit test
Reviewer: @artemcm @xymus