Support converting simple $(eval) expressions

While supporting $(eval) in the general case is
impossible, as it would require emitting code at
runtime, it is possible to handle some special cases
that are common throughout the code base.

Specifically, an assignement expression (where the
left hand side is constant) can be converted without
needing to evaluate strings as code, as its whole
right hand side is treated as one string.

However, this eval with an assignemnt can only be
used as a statement, not an expression. So it requires
the eval to be either a top-level expression, or nested
within other expressions that can be converted to
statements such as $(foreach) or $(if).

Bug: 226974242
Test: go test
Change-Id: Ifc52ef9ab7d62a69251918fcde5463f80a98a542