[frontend] 4.0: Add 'finishProcessing()' method for DiagnosticConsumers to do their finalization (e.g. writing to a file)

[akyrtzi]

Use this to avoid creating an empty serialized diagnostics file when the compiler crashes.

rdar://31909195

master: #9222

[akyrtzi]

@swift-ci Please test

[akyrtzi]

Explanation: The Swift compiler creates the serialized diagnostics file at the beginning and if it crashes it leaves an empty file for the serialized diagnostics. When Xcode tries to read the file it uses a libclang API which crashes if the file is empty (it considers it malformed). This change makes the Swift compiler to open the serialized diagnostics file when it is read to write to it at the end, so if the compiler crashes it doesn't leave an empty file. Making the libclang API more robust is tracked via a different radar.
Scope: Affects when the compiler creates the serialized diagnostics file
Radar: rdar://31909195
Risk: Low
Testing: Added regression test to make sure there's no diagnostics file emitted if there is a compiler crash