[clang][dataflow] Add `Strict` versions of `Value` and `StorageLocation` accessors.

This is part of the gradual migration to strict handling of value categories, as described in the RFC at https://discourse.llvm.org/t/70086.

This patch migrates some representative calls of the newly deprecated accessors to the new `Strict` functions. Followup patches will migrate the remaining callers.  (There are a large number of callers, with some subtlety involved in some of them, so it makes sense to split this up into multiple patches rather than migrating all callers in one go.)

The `Strict` accessors as implemented here have some differences in semantics compared to the semantics originally proposed in the RFC; specifically:

*  `setStorageLocationStrict()`: The RFC proposes to create an intermediate
   `ReferenceValue` that then refers to the `StorageLocation` for the glvalue.
   It turns out though that, even today, most places in the code are not doing
   this but are instead associating glvalues directly with their
   `StorageLocation`. It therefore didn't seem to make sense to introduce new
   `ReferenceValue`s where there were none previously, so I have chosen to
   instead make `setStorageLocationStrict()` simply call through to
   `setStorageLocation(const Expr &, StorageLocation &)` and merely add the
   assertion that the expression must be a glvalue.

*  `getStorageLocationStrict()`: The RFC proposes that this should assert that
   the storage location for the glvalue expression is associated with an
   intermediate `ReferenceValue`, but, as explained, this is often not true.
   The current state is inconsistent: Sometimes the intermediate
   `ReferenceValue` is there, sometimes it isn't. For this reason,
   `getStorageLocationStrict()` skips past a `ReferenceValue` if it is there but
   otherwise directly returns the storage location associated with the
   expression. This behavior is equivalent to the existing behavior of
   `SkipPast::Reference`.

*  `setValueStrict()`: The RFC proposes that this should always create the same
   `StorageLocation` for a given `Value`, but, in fact, the transfer functions
   that exist today don't guarantee this; almost all transfer functions
   unconditionally create a new `StorageLocation` when associating an expression
   with a `Value`.

   There appears to be one special case:
   `TerminatorVisitor::extendFlowCondition()` checks whether the expression is
   already associated with a `StorageLocation` and, if so, reuses the existing
   `StorageLocation` instead of creating a new one.

   For this reason, `setValueStrict()` implements this logic (preserve an
   existing `StorageLocation`) but makes no attempt to always associate the same
   `StorageLocation` with a given `Value`, as nothing in the framework appers to
   require this.

   As `TerminatorVisitor::extendFlowCondition()` is an interesting special case,
   the `setValue()` call there is among the ones that this patch migrates to
   `setValueStrict()`.

Reviewed By: sammccall, ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D150653

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