transpile: Conservatively disallow mutable lvalues inside macros

[Rua]

• Depends on transpile: Fix indentation in macros.c snapshot test #1993

Mutable lvalues are values that can either be assigned to, or have a mutable pointer created to them. If they are in a macro, that could pose problems. DeclRefs (including those to mutable variables) are already disallowed in macros, but another "out of nothing" source of mutable lvalues is compound literals. Currently, a macro containing a mutable compound literal is translated as-is, including something like this:

#define MUTATE_ME (&(int){ 42 })

int *p1 = MUTATE_ME;
int *p2 = MUTATE_ME;

Each mutable compound literal expression is supposed to create its own distinct memory location (immutable ones are allowed to be coalesced). So these pointers must be different, and their pointees must be able to be modified independently.

I've been very conservative here and disallowed all mutable lvalues in macros. There are certainly instances that could be re-allowed on a case-by-case basis. Certain instances of lvalues that undergo an LValueToRValue cast for example.