build-llvm: Implicitly set LLVM_USE_RELATIVE_PATHS_IN_FILES when using ccache

This is an option I set outside of this script in a number of
build cases where I build with ccache; move this into the script.

In practice it has little effect though; one has to set
CCACHE_BASEDIR anyway in order to be able to share a ccache
between build directories.

If enabling ccache by adding e.g. /usr/lib/ccache to the front of
PATH, without using the COMPILER_LAUNCHER variable, this won't
get enabled.

build-llvm.sh

@@ -240,6 +240,13 @@ fi
 if [ -n "$COMPILER_LAUNCHER" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER_LAUNCHER=$COMPILER_LAUNCHER"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER_LAUNCHER=$COMPILER_LAUNCHER"
+    # Make the LLVM build system set options for forcing relative paths
+    # within the files, e.g. for source file references within assert
+    # messages. This on its own isn't enough for making the cache reusable
+    # across different worktrees though; one also needs to set the ccache
+    # base_dir (CCACHE_BASEDIR) option. When setting that ccache option, this
+    # option here doesn't really have any effect either, except for debug info.
+    CMAKEFLAGS="$CMAKEFLAGS -DLLVM_USE_RELATIVE_PATHS_IN_FILES=ON"
 fi
 
 if [ -n "$LTO" ]; then