[analyzer] LValueToRValueBitCasts should evaluate to an r-value

Previously `LValueToRValueBitCast`s were modeled in the same way how
a regular `BitCast` was. However, this should not produce an l-value.
Modeling bitcasts accurately is tricky, so it's probably better to
model this expression by binding a fresh conjured value.

The following code should not result in a diagnostic:
```lang=C++
  __attribute__((always_inline))
  static inline constexpr unsigned int_castf32_u32(float __A) {
    return __builtin_bit_cast(unsigned int, __A); // no-warning
  }
```

Previously, it reported
`Address of stack memory associated with local variable '__A' returned
to caller [core.StackAddressEscape]`.

Differential Revision: https://reviews.llvm.org/D105017

Reviewed by: NoQ, vsavchenko