Bump to LLVM 18.1.0 (#393)

* Explicitly set LIBCXXABI_USE_LLVM_UNWINDER off

Clang trunk has recently turned it on, and also now fails the build when
libunwind is not built at the same time.

* Bump LLVM to 18.1.0

---------

Co-authored-by: Mike Hommey <[email protected]>

Author: alexcrichton

Makefile

@@ -81,7 +81,6 @@ build/llvm.BUILT:
 		-DLLVM_ENABLE_ZLIB=OFF \
 		-DLLVM_ENABLE_ZSTD=OFF \
 		-DLLVM_STATIC_LINK_CXX_STDLIB=ON \
-		-DLLVM_HAVE_LIBXAR=OFF \
 		-DCMAKE_INSTALL_PREFIX=$(PREFIX) \
 		-DLLVM_INCLUDE_TESTS=OFF \
 		-DLLVM_INCLUDE_UTILS=OFF \
@@ -212,6 +211,7 @@ LIBCXX_CMAKE_FLAGS = \
     -DLIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY:BOOL=OFF \
     -DLIBCXXABI_HAS_WIN32_THREAD_API:BOOL=OFF \
     -DLIBCXXABI_ENABLE_PIC:BOOL=$(2) \
+    -DLIBCXXABI_USE_LLVM_UNWINDER:BOOL=OFF \
     -DWASI_SDK_PREFIX=$(BUILD_PREFIX) \
     -DUNIX:BOOL=ON \
     --debug-trycompile \