Determine checked scopes per statement

[secure-sw-dev-bot]

This issue was copied from microsoft/checkedc-clang#1142

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We introduce a 2-bit field called CheckedScopeSpecifier in the Stmt class.
During parsing when a compound statement is created we iterate the elements
(statements) of the compound statement and set the checked scope specifier for
each element to the checked scope specifier of the compound statement.

We can get the checked scope specifier for a statement by calling the
getCheckedScopeSpecifier method on the statement.

[secure-sw-dev-bot]

Comment from @mgrang:

Depends on microsoft/checkedc-clang#1141