[llvm] MC: Restructure MCFragment as a fixed part and a variable tail (PR #148544)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I apologize for any disruption in the MC layer. The integrated assembler was previously poorly designed and very inefficient. Over the past two years, I've significantly overhauled the relocation generation process.
A few MCFragment-related changes are still pending, but I expect stabilization soon, despite working on this in my limited spare time. These improvements have also made target support much cleaner.
I believe adding non-blocking reviewers is generally reserved for individuals with prior contributions, such as code or review, rather than solely based on their usage of the project.
As an alternative, subscribing to labels is a generic mechanism provided by github :)

https://github.com/llvm/llvm-project/pull/148544