Add clangd config file (#249)

### What problem were solved in this pull request?
Problem:
clangd default finds `compile_commands.json` in `build` directory and
the miniob's default building directory is not `build` which will lead
to the clangd's failure of searching `compile_commands.json`.

### What is changed and how it works?

create a soft link from build to build_xxx in build.sh when building.

.gitignore

@@ -10,6 +10,8 @@
 build/*
 build_*
 cmake-build-*/*
+build
+.cache/*
 .DS_Store
 .idea
 .ccls-cache

build.sh

@@ -114,7 +114,11 @@ function do_init
 function prepare_build_dir
 {
   TYPE=$1
-  mkdir -p ${TOPDIR}/build_${TYPE} && cd ${TOPDIR}/build_${TYPE}
+  mkdir -p ${TOPDIR}/build_${TYPE}
+  rm -f build
+  echo "create soft link for build_${TYPE}, linked by directory named build"
+  ln -s build_${TYPE} build
+  cd ${TOPDIR}/build_${TYPE}
 }
 
 function do_build