stage2: implement function call inlining in the frontend

 * remove the -Ddump-zir thing. that's handled through --verbose-ir
 * rework Fn to have an is_inline flag without requiring any more memory
   on the heap per function.
 * implement a rough first version of dumping typed zir (tzir) which is
   a lot more helpful for debugging than what we had before. We don't
   have a way to parse it though.
 * keep track of whether the inline-ness of a function changes because
   if it does we have to go update callsites.
 * add compile error for inline and export used together.

inline function calls and comptime function calls are implemented the
same way. A block instruction is set up to capture the result, and then
a scope is set up that has a flag for is_comptime and some state if the
scope is being inlined.

when analyzing `ret` instructions, zig looks for inlining state in the
scope, and if found, treats `ret` as a `break` instruction instead, with
the target block being the one set up at the inline callsite.

Follow-up items:
 * Complete out the debug TZIR dumping code.
 * Don't redundantly generate ZIR for each inline/comptime function
   call. Instead we should add a new state enum tag to Fn.
 * comptime and inlining branch quotas.
 * Add more test cases.

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