Where is the profile-gc instrumentation inserted

[Teodor Dutu]

Hi,

I have recently finished [translating `_d_arraycatnTX()` to a 
template](https://github.com/dlang/dmd/pull/14550). While 
cleaning up the code, I was unable to figure out why `profile-gc` 
also shows the new template hook in addition to its former 
output. This behaviour is consistent on all `profile-gc` tests in 
DRuntime. One example is [this 
one](https://github.com/dlang/dmd/blob/4db85299eba70416c2f508568aec76e430fcb575/druntime/test/profile/myprofilegc.log.linux.64.exp), which I had to modify by adding the line with `_d_arraycatnTX()`:

```text

bytes allocated, allocations, type, function, file:line
             560	              3	int[] 
core.internal.array.concatenation._d_arraycatnTX!(int[], string, 
int, string, int[], int[])._d_arraycatnTX 
../../src/core/internal/array/concatenation.d:42
             464	              1	immutable(char)[][int] D main 
src/profilegc.d:23
             160	              1	float[][] D main 
src/profilegc.d:18
             160	              1	int[][] D main src/profilegc.d:15
              64	              1	double[] profilegc.main 
src/profilegc.d:56
              48	              1	float[] D main src/profilegc.d:42
              48	              1	int[] D main src/profilegc.d:41
              32	              1	profilegc.main.C D main 
src/profilegc.d:12
              32	              1	void[] profilegc.main 
src/profilegc.d:55
              16	              1	char[] D main src/profilegc.d:34
              16	              1	char[] D main src/profilegc.d:36
              16	              1	closure profilegc.main.foo 
src/profilegc.d:45
              16	              1	float D main src/profilegc.d:16
              16	              1	float[] D main src/profilegc.d:17
              16	              1	int D main src/profilegc.d:13
              16	              1	int[] D main src/profilegc.d:14
              16	              1	int[] D main src/profilegc.d:22
              16	              1	int[] D main src/profilegc.d:37
              16	              1	wchar[] D main src/profilegc.d:35
```

My assumption is that the GC's profiling instrumentation is 
introduced in the semantic phase, before the new lowering, thus 
also taking the `_d_arraycatnTX` hook into account. If this is 
true, then the old lowering was not profiled because it took 
place in the glue layer, after the instrumentation had been 
introduced.

Regarding this, I have 3 questions that I don't know how to 
answer:
1. Is my assumption above correct?
1. Are those 560 bytes an issue that means the new lowering 
over-allocates 560 bytes? The value is the same regardless of 
platform and OS.
1. Is it OK for the output to contain an "internal" function, 
such as a runtime hook?

What do you think?

Thanks,
Teo