build-llvm: Remove the --symlink-projects option

This option was mostly relevant for making it possible to share
ccache between builds in different directories, due to specific
logic in CMake's Ninja generator, which used relative paths for
sources as long as they were below the main source directory
(llvm-project/llvm). This option wasn't usable on Windows.

Since CMake 3.21.0, this option no longer worked for the intended
purpose - that version of CMake changed the Ninja generator to
always use absolute paths to sources, for consistency with other
generators and to simplify things. (See commit
c564a3e3fff52ef811291c5ba8fb07a5a1b47f97 in CMake's git history.)

It is still possible to achieve ccache sharing across build
directories anyway, by setting the ccache option CCACHE_BASEDIR,
which makes ccache rewrite absolute paths to relative ones within
that tree, and by setting the LLVM CMake option
LLVM_USE_RELATIVE_PATHS_IN_FILES=ON, which adds -ffile-prefix-map
options to the compiler, to remap source paths as they are exposed
in the generated code.

(Strictly speaking, LLVM_USE_RELATIVE_PATHS_IN_FILES is only needed
if debug info is enabled. On its own, without CCACHE_BASEDIR, that
option produces compiler output that is identical across build
directories - but the absolute paths end up in the preprocessor output
in source file/line comments, which makes ccache fail to reuse the cache.)

build-llvm.sh

@@ -59,9 +59,6 @@ while [ $# -gt 0 ]; do
     --with-python)
         WITH_PYTHON=1
         ;;
-    --symlink-projects)
-        SYMLINK_PROJECTS=1
-        ;;
     --disable-lldb)
         unset LLDB
         ;;
@@ -77,7 +74,7 @@ done
 BUILDDIR="$BUILDDIR$ASSERTSSUFFIX"
 if [ -z "$CHECKOUT_ONLY" ]; then
     if [ -z "$PREFIX" ]; then
-        echo $0 [--enable-asserts] [--stage2] [--thinlto] [--lto] [--disable-dylib] [--full-llvm] [--with-python] [--symlink-projects] [--disable-lldb] [--disable-clang-tools-extra] [--host=triple] dest
+        echo $0 [--enable-asserts] [--stage2] [--thinlto] [--lto] [--disable-dylib] [--full-llvm] [--with-python] [--disable-lldb] [--disable-clang-tools-extra] [--host=triple] dest
         exit 1
     fi
 
@@ -283,41 +280,12 @@ fi
 
 cd llvm-project/llvm
 
-if [ -n "$SYMLINK_PROJECTS" ]; then
-    # If requested, hook up other tools by symlinking them into tools,
-    # instead of using LLVM_ENABLE_PROJECTS. This way, all source code is
-    # under the directory tree of the toplevel cmake file (llvm-project/llvm),
-    # which makes cmake use relative paths to all source files. Using relative
-    # paths makes for identical compiler output from different source trees in
-    # different locations (for cases where e.g. path names are included, in
-    # assert messages), allowing ccache to share caches across multiple
-    # checkouts.
-    cd tools
-    for p in clang lld lldb; do
-        if [ "$p" = "lldb" ] && [ -z "$LLDB" ]; then
-            continue
-        fi
-        if [ ! -e $p ]; then
-            ln -s ../../$p .
-        fi
-    done
-    cd ..
-    if [ -n "$CLANG_TOOLS_EXTRA" ]; then
-        cd ../clang/tools
-        if [ ! -e extra ]; then
-            ln -s ../../clang-tools-extra extra
-        fi
-        cd ../../llvm
-    fi
-else
-    EXPLICIT_PROJECTS=1
-    PROJECTS="clang;lld"
-    if [ -n "$LLDB" ]; then
-        PROJECTS="$PROJECTS;lldb"
-    fi
-    if [ -n "$CLANG_TOOLS_EXTRA" ]; then
-        PROJECTS="$PROJECTS;clang-tools-extra"
-    fi
+PROJECTS="clang;lld"
+if [ -n "$LLDB" ]; then
+    PROJECTS="$PROJECTS;lldb"
+fi
+if [ -n "$CLANG_TOOLS_EXTRA" ]; then
+    PROJECTS="$PROJECTS;clang-tools-extra"
 fi
 
 [ -z "$CLEAN" ] || rm -rf $BUILDDIR
@@ -329,7 +297,7 @@ cmake \
     -DCMAKE_INSTALL_PREFIX="$PREFIX" \
     -DCMAKE_BUILD_TYPE=Release \
     -DLLVM_ENABLE_ASSERTIONS=$ASSERTS \
-    ${EXPLICIT_PROJECTS+-DLLVM_ENABLE_PROJECTS="$PROJECTS"} \
+    -DLLVM_ENABLE_PROJECTS="$PROJECTS" \
     -DLLVM_TARGETS_TO_BUILD="ARM;AArch64;X86" \
     -DLLVM_INSTALL_TOOLCHAIN_ONLY=$TOOLCHAIN_ONLY \
     -DLLVM_LINK_LLVM_DYLIB=$LINK_DYLIB \