Save normalized VarDecl bounds

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#841

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Fixes #830

This PR attaches a normalized bounds expression to a VarDecl to avoid repeated calls to ExpandBoundsToRange, which can allocate AST structures and increase the memory usage of the compiler. This is relevant for updating and validating the observed context and using widened bounds, which is currently recomputing the normalized bounds of a VarDecl.

Testing:

• Passed manual testing on Windows
• Passed automated testing on Linux

[secure-sw-dev-bot]

Comment from @kkjeer:

Do you plan to cleanup the invocations of ExpandBoundsToRange in other files as a follow up?

Created #848 to track this.