[clang] Support for read-only types

The main goal of this work is to allow developers to express the need to place instances
of a class or structure in the read-only part of the program memory. Such a placement is
desirable to prevent any further modifications to the instances of a given structure, by
leveraging the read-only run time protection.

To achieve this, we are introducing a new attribute that can be attached to any record
definition or a declaration. The compiler enforces that every instance of this type can
be placed in the read-only segment of the program memory, provided the target triplet
supports such a placement. If an instance of a given type bearing this attribute doesn’t
satisfy such a placement, the compiler attaches an appropriate warning at suitable program
locations. In other words, adding this attribute to a type requires every instance of this
type to be a global const, which are placed in the read-only segments for most target
triplets. However, this is *not a language feature* and it *need not* be true for
*all target triplets*.

The current patch emits a warning at global variable declaration sites of types bearing
the attribute without const qualification and corresponding note attached to the type
definition/declaration.

Differential Revision: https://reviews.llvm.org/D135851

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