[clang][dataflow] Don't associate prvalue expressions with storage locations.

Instead, map prvalue expressions directly to values in a newly introduced map `Environment::ExprToVal`.

This change introduces an additional member variable in `Environment` but is an overall win:

- It is more conceptually correctly, since prvalues don't have storage
  locations.

- It eliminates complexity from
  `Environment::setValue(const Expr &E, Value &Val)`.

- It reduces the amount of data stored in `Environment`: A prvalue now has a
  single entry in `ExprToVal` instead of one in `ExprToLoc` and one in
  `LocToVal`.

- Not allocating `StorageLocation`s for prvalues additionally reduces memory
  usage.

This patch is the last step in the migration to strict handling of value categories (see https://discourse.llvm.org/t/70086 for details). The changes here are almost entirely internal to `Environment`.

The only externally observable change is that when associating a `RecordValue` with the location returned by `Environment::getResultObjectLocation()` for a given expression, callers additionally need to associate the `RecordValue` with the expression themselves.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D158977