build-llvm: Bump to LLVM 20.1.0 RC 2

Relevant changes in LLVM 20.1.x compared with LLVM 19.x are:

- Don't warn when doing LTO with mixed Rust/C object files, where
  the target triples differ in the vendor field
  (<arch>-w64-windows-gnu vs <arch>-pc-windows-gnu)
- Fix including LLVM/MSVC style import libraries in libraries linked
  with --whole-archive
- Support for files over 2 GB in the libcxx fstream methods
  tellg()/seekg()
- Lots of work on improved ARM64EC and ARM64X support in LLVM and LLD
- New linker flags --dependent-load-flag and --functionpadmin, similar
  to the corresponding lld-link/MS link.exe flags
- Improvements in address sanitizer, supporting many more x86
  instructions
- Changes to address sanitizer, that (unintentionally) makes it fail
  harder than before when used together with msvcrt.dll. Using
  address sanitizer with msvcrt.dll is unsupported though.

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-20.1.0-rc1}
+: ${LLVM_VERSION:=llvmorg-20.1.0-rc2}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"