This adds an `emitc.lvalue` type which models assignable lvlaues in the
type system. Operations modifying memory are restricted to this type
accordingly.

See also the discussion on
[discourse](https://discourse.llvm.org/t/rfc-separate-variables-from-ssa-values-in-emitc/75224/9).
The most notable changes are as follows.

- `emitc.variable` and `emitc.global` ops are restricted to return
`emitc.array` or `emitc.lvalue` types
- Taking the address of a value is restricted to operands with lvalue
type
- Conversion from lvalues into SSA values is done with the new
`emitc.load` op
- The var operand of the `emitc.assign` op is restricted to lvalue type 
- The result of the `emitc.subscript` and `emitc.get_global` ops is a
lvalue type
- The operands and results of the `emitc.member` and
`emitc.member_of_ptr` ops are restricted to lvalue types

---------

Co-authored-by: Matthias Gehre <[email protected]>