[clang][dataflow] Fix crash when RHS of `&&` or `||` calls `noreturn` func.

[clang][dataflow] Fix crash when RHS of `&&` or `||` calls `noreturn` func.

The crash happened because the transfer fucntion for `&&` and `||`
unconditionally tried to retrieve the value of the RHS. However, if the RHS
is unreachable, there is no environment for it, and trying to retrieve the
operand's value causes an assertion failure.

See also the comments in the code for further details.

Reviewed By: xazax.hun, ymandel, sgatev, gribozavr2

Differential Revision: https://reviews.llvm.org/D146514