[circleci] Use package managers to get the lit env dependencies.

This makes the dependencies handling less manual for lit and FileCheck.

.circleci/build.sh

@@ -24,6 +24,14 @@ install_pocl() {
    cd ../
 }
 
+install_lit_env() {
+    sudo apt-get install -y llvm-6.0-tools
+    # We could also use `pip install lit` if the package didn't come with
+    # lit.py.
+    export PATH="$PATH:/usr/lib/llvm-6.0/build/utils/lit:/usr/lib/llvm-6.0/bin"
+    hash lit.py FileCheck
+}
+
 # setup sccache wrappers
 if hash sccache 2>/dev/null; then
     SCCACHE_BIN_DIR="/tmp/sccache"
@@ -63,7 +71,7 @@ if [[ "${CIRCLE_JOB}" == "ASAN" ]]; then
     CMAKE_ARGS+=("-DCMAKE_BUILD_TYPE=Release")
 elif [[ "$CIRCLE_JOB" == RELEASE_WITH_LIT ]]; then
     # Setup the LLVM Integrated Tester if requested.
-    source $GLOW_DIR/utils/set_llvm_test_env.sh
+    install_lit_env
 
     # Download the models that LIT runs and tell cmake where to find them.
     MODELS_DIR="$GLOW_DIR/downloaded_models"

CMakeLists.txt

@@ -101,7 +101,12 @@ else()
 endif()
 
 find_program(FILECHECK "FileCheck")
+# The lit environment works equally with lit.py from llvm source or
+# the python lit package. Check for the other one if lit.py failed.
 find_program(LLVM_LIT "lit.py")
+if (NOT LLVM_LIT)
+  find_program(LLVM_LIT "lit")
+endif()
 
 if (FILECHECK AND LLVM_LIT)
   set(HAS_LLVM_TEST_ENV ON)