Sema: rewrite semantic analysis of function calls

This rewrite improves some error messages, hugely simplifies the logic,
and fixes several bugs. One of these bugs is technically a new rule
which Andrew and I agreed on: if a parameter has a comptime-only type
but is not declared `comptime`, then the corresponding call argument
should not be *evaluated* at comptime; only resolved. Implementing this
required changing how function types work a little, which in turn
required allowing a new kind of function coercion for some generic use
cases: function coercions are now allowed to implicitly *remove*
`comptime` annotations from parameters with comptime-only types. This is
okay because removing the annotation affects only the call site.

Resolves: #22262

test/behavior/eval.zig

@@ -363,7 +363,7 @@ test "comptime modification of const struct field" {
 }
 
 test "refer to the type of a generic function" {
-    const Func = fn (type) void;
+    const Func = fn (comptime type) void;
     const f: Func = doNothingWithType;
     f(i32);
 }