Add AST Serialization/Deserialization Tests

[lenary]

This is making sure that we don't break pre-compiled headers at any point.

Not only do we include new PCH tests, but in order to get them to pass, I had to fix the following bugs:

• One of the initializers of PositionalParameterExpr (specifically the Empty one) had a copy/paste bug, which meant it was initializing the wrong kind of expression.
• We were not writing the sub-expressions within bounds expressions correctly. Now they are written in the same way that the visitor writes other sub-expressions of other expression types.
• We were not saving the "type as written" information for the InteropTypeBoundsAnnotationExpr, which meant that we couldn't deserialize it. It is now saved and re-read correctly.

The test compiles a header and source file together, then tests compiling the header separately into a PCH, and then compiling the source file using the PCH. They should have the same set of errors. We use -verify and diagnostic annotations as tests. Currently we ignore diagnostic notes, as there seems to be an issue with putting expectations in the header to be compiled into the PCH.

Currently most of the tests are testing that everything works, rather than testing that certain things fail. This could be improved later.

This should help us with #3.

[msftclas]

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[dtarditi]

The changes look good and are approved for commit, with one requested change. Please modify the comments for pch.c to describe the purpose of the test (something along the lines of "This file tests pre-compiled headers that use the Checked C extension.").

As a follow up item, could you add additional tests for Ptr types, function pointer types, checked arrays, and structures with checked members that have bounds declarations? I would like to test that those types are serialized properly.