Integrated assembler improvements in LLVM 19

Nice summary! Additional changes I have planned:

- Removing per-instruction timers, which add a measurable overhead even when disabled (https://github.com/llvm/llvm-project/pull/97046)

- Splitting AsmPrinterHandler (used for unwind info) and DebugHandler (used also for per-instruction location information) to avoid two virtual function calls per instruction (https://github.com/llvm/llvm-project/pull/96785)

- Remove several maps from ELFObjectWriter, including some std::map (changed locally, need to make PR)

- Faster section allocation, remove ELF "mergeable section info" hash maps (although this is called just ~40 times per object file, it is very measurable in JIT use cases when compiling many small objects) (planned)

- X86 encoding in general; this consumes quite some time and looks very inefficient -- having written my own x86 encoder, I'm confident that there's a lot of improvement potential. (not started)

Some takeaways on a higher level -- most of these aren't really surprising, but nonetheless are very frequent problems(/patterns) in the LLVM code base:

- Maps/hash maps/sets are quite expensive when used frequently, and sometimes can be easily avoided, e.g., with a vector or, for pointer keys, a pointer dereference

- Virtual functions(/abstraction) calls comes at a cost, especially when done frequently

- raw_svector_ostream is slow, because writes are virtual function calls and don't get inlined (I previously replaced raw_svector_ostream with a SmallVector&: https://reviews.llvm.org/D145792)

- Frequent heap allocations are costly, especially with glibc's malloc

- Many small inefficiencies add up (=> many small improvements do, too)

Side note, but I was looking for a pre-built binaries in releases of LLVM project. Specifically I was looking for clang+llvm releases for x86_64 linux (ubuntu preferably) in order to save some time (always had trouble compiling it) and to put it into my own `prefix` directory. It's kind of wild to see aarch64, armv7, powerpc64, x86_64_windows.. but not something like this. I am aware of https://apt.llvm.org/ and its llvm.sh - but as I said, I'd prefer it to live in its own `prefix`. Anyone knows where else there might be pre-builts? There used to be something just like that for v17, like https://github.com/llvm/llvm-project/releases/download/llvmo...

In the first sentence, "[Intro to the LLVM MC Project]" was likely intended to be a link[1].

[1] https://blog.llvm.org/2010/04/intro-to-llvm-mc-project.html

TLDR: building projects with Clang is now about 4% faster due to optimizations in the way it internally handles assembly.