Parse bounds declarations for function return values.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#21

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This extends parsing of function declarators to parse bounds expressions for
function return values. A return bounds expression is declared by following
a parameter list with ':' bounds-expression.

The return bounds information is propagated to the representation of function
declarations in clang. It is not propagated yet to function types because
there is no way to represent it in function types. I have opened issue #20
to track that and added a few TODO's to the code flagging where the
information will need to be added.

Testing:

• Add feature tests for parsing return bounds. The tests are in
  the file return_bounds.c, which will be committed separately to the checkedc
  Github repo.
• Passes existing clang regression test baseline.

[secure-sw-dev-bot]

Comment from @msftclas:

Hi @dtarditi, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

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[secure-sw-dev-bot]

Comment from @reubeno:

I called out a few very low-priority suggestions; otherwise, looks great.

[secure-sw-dev-bot]

Comment from @awruef:

Looks good to me.