[LLVM] Automatically strip -fno-lifetime-dse from compile_commands.json

Summary:
This flag is always passed by LLVM to bypass some LTO related bug with
GCC. The problem is that this is not supported by clang and shows up in
the compile_commands.json file. That means it will always show at least
one error complaining about this file. Since I already have a script to
merge the runtimes stuff, just strip this out there for convenience.

There was a discussion a long time ago about not adding this as a no-op
flag to clang, so this is the next best thing.

Author: jhuber6

llvm/utils/merge-json.py

@@ -35,6 +35,13 @@ def main():
         except (IOError, json.JSONDecodeError) as e:
             continue
 
+    # LLVM passes this argument by default but it is not supported by clang,
+    # causing annoying errors in the generated compile_commands.json file.
+    # Remove it here before we deduplicate the entries.
+    for entry in merged_data:
+        if isinstance(entry, dict) and "command" in entry:
+            entry["command"] = entry["command"].replace("-fno-lifetime-dse ", "")
+
     # Deduplicate by converting each entry to a tuple of sorted key-value pairs
     unique_data = list({json.dumps(entry, sort_keys=True) for entry in merged_data})
     unique_data = [json.loads(entry) for entry in unique_data]