Fix run_clang_repl output when not present (llvm#161412)

On the happy path, when `clang-repl` is present, we will invoke it in
order to determine if the host supports JIT features. That will return a
string containing "true". However, in cases where `clang-repl` is not
present or we fail to invoke it, we previously returned `False`, which
would then trigger a failure with our substring check. This PR updates
the function to return `""` instead, so the substring check is still
valid.

This is related to llvm#157359,
where the original change was introduced.

Author: jeaye

clang/test/lit.cfg.py

@@ -145,15 +145,15 @@ def run_clang_repl(args):
     clang_repl_exe = lit.util.which("clang-repl", config.clang_tools_dir)
 
     if not clang_repl_exe:
-        return False
+        return ""
 
     try:
         clang_repl_cmd = subprocess.Popen(
             [clang_repl_exe, args], stdout=subprocess.PIPE
         )
     except OSError:
         print("could not exec clang-repl")
-        return False
+        return ""
 
     clang_repl_out = clang_repl_cmd.stdout.read().decode("ascii")
     clang_repl_cmd.wait()