build-llvm: Update to llvm 15.0.0 RC1

Notable changes are:
- Fix stripping Rust executables with .rmeta sections
- Implemented support for SEH unwind info on the armv7 target
- Fix compiling files specified with an absolute \\?\C:\... style path
- Implemented support for the --disable-reloc-section option in lld
- Lots of improvements in LLDB on Windows (thanks to Alvin Wong!)
 - Support for GNU debuglink
 - Fix loading DLLs from a ramdisk
 - Fix starting executables without including the '.exe' suffix
 - Fixed crashes on getting nested exceptions
 - Use the right C++ ABI for mingw environments

Other changes that simplify how to build the toolchain:
- Improvements to how libcxx/libcxxabi/libunwind are built for Windows
  (and in particular mingw), allowing building both shared and static
  libraries in the same cmake build
- Avoids needing to set an empty CROSS_TOOLCHAIN_FLAGS_NATIVE when
  cross compiling LLVM

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-14.0.0}
+: ${LLVM_VERSION:=llvmorg-15.0.0-rc1}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"