[clang] [clang-tools-extra] [compiler-rt] [lldb] [llvm] [NFCI][RTTI][Passes] Make the ID fields const char (PR #179970)

[Steffen Larsen via cfe-commits]

steffenlarsen wrote:

> Does LLVM rely on each ID field having a unique address?

Based on my understanding, that is the only use-case of it. Either for RTTI or for legacy pass identification.

> Does making them const mean the linker could common them up so they no longer have unique addresses?

The [C++ Standard (draft) [intro.object] §10](https://eel.is/c++draft/intro.object#10) says:

> Unless an object is a bit-field or a subobject of zero size, the address of that object is the address of the first byte it occupies. Two objects with overlapping lifetimes that are not bit-fields may have the same address if
>  * one is nested within the other,
>  * at least one is a subobject of zero size and they are not of similar types ([conv.qual]), or
>  * they are both potentially non-unique objects;
> otherwise, they have distinct addresses and occupy disjoint bytes of storage.

Since it's neither a subobject nor zero-sized, they should supply unique addresses. The example in the paragraph also shows a similar case.

> Have you seen this getting misused upstream or in a downstream project?

I haven't seen it happen, but to give some additional context: I'm having a look at what would be required to safely use Clang and LLVM libraries in a multi-threaded context. In such a case, any mutable global state is a risk. Having the `ID` be `const` seemed to fit the intention of it, so I decided to get ahead of the curve and propose the change now.

https://github.com/llvm/llvm-project/pull/179970