When calculating `StringlikeLiteral.getValueText`, include results from
interpolations where we can determine their string value. For example:
b = "b" # local variable
D = "d" # constant
"a#{b}c" # getValueText() = "abc"
"a#{b}c{D}" # getValueText() = "abcd"
/#a#{b}c{D}/ # getValueText() = "abcd"