[clang][dataflow] Clear `ExprToLoc` and `ExprToVal` at the start of a block. (#72985)

We never need to access entries from these maps outside of the current
basic
block. This could only ever become a consideration when flow control
happens
inside a full-expression (i.e. we have multiple basic blocks for a full
expression); there are two kinds of expression where this can happen,
but we
already deal with these in other ways:

* Short-circuiting logical operators (`&&` and `||`) have operands that
live in
different basic blocks than the operator itself, but we already have
code in
the framework to retrieve the value of these operands from the
environment
for the block they are computed in, rather than in the environment of
the
   block containing the operator.

* The conditional operator similarly has operands that live in different
basic
blocks. However, we currently don't implement a transfer function for
the
conditional operator. When we do this, we need to retrieve the values of
...