build-llvm: Bump to LLVM 18.1.0 RC 1

Relevant changes/fixes since the last version:
- Added the new Clang option -fno-auto-import, affecting both code
  generation (avoiding generating .refptr stubs) and linking
  (passing --disable-auto-import, to avoid subtle runtime errors,
  which could happen if code built without .refptr stubs would
  autoimport symbols anyway)
- Fix issues with running the toolchain, or building sources on,
  network mounts. Previously, there were issues with VirtualBox
  shared folders or Remote Desktop shared folders.
- Pass LTO options from the compiler driver to the linker
- Support more LTO options in the linker
- LLD now implements the --icf=safe option properly
- Support using the -S/-s flags in LLD, to strip out DWARF debug
  info and symbol table, while producing a PDB file
- Fix LTO issues around __imp_ symbols, making it support building
  the base mingw-w64-crt files with LTO
- LLD now doesn't try to detect/add MSVC library directories when
  linking in MinGW mode
- Fix building compiler-rt sanitizers, when libcxx is available only
  in static form
- Fix building a standalone libclang_rt.atomic_dynamic for MinGW
  targets
- Support for the MENUEX resource type in llvm-windres
- llvm-windres now supports referencing filenames in .rc files,
  constructed from multiple concatenated string literals
- Various fixes for the llvm-windres option --preprocessor, including
  the breaking change to match the new GNU windres behaviour from
  binutils 2.36
- Don't look for a GCC in path when Clang is installed next to
  a suitable sysroot (mostly relevant for MSYS2)
- Avoid exposing unprefixed mmap symbols in the profiling runtime,
  which could lead to misdetections in user code
- Support for reading Windows thread names in LLDB

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-17.0.6}
+: ${LLVM_VERSION:=llvmorg-18.1.0-rc1}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"