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/generics.zig

@@ -427,7 +427,7 @@ test "generic function passed as comptime argument" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
 
     const S = struct {
-        fn doMath(comptime f: fn (type, i32, i32) error{Overflow}!i32, a: i32, b: i32) !void {
+        fn doMath(comptime f: fn (comptime type, i32, i32) error{Overflow}!i32, a: i32, b: i32) !void {
             const result = try f(i32, a, b);
             try expect(result == 11);
         }