[lldb][fuzz] Allow expression fuzzer to be passed as a flag.

The expression fuzzer checks an environment variable, `LLDB_FUZZER_TARGET`, to get the fuzzer target binary. This is fine, but internally our tooling for running fuzz tests only has proper handling for flag values. It's surprisingly complicated to add support for that, and allowing it to be passed via flag seems reasonable anyway.

Reviewed By: cassanova

Differential Revision: https://reviews.llvm.org/D133546

Author: rupprecht

lldb/tools/lldb-fuzzer/lldb-expression-fuzzer/lldb-expression-fuzzer.cpp

@@ -35,7 +35,7 @@ using namespace lldb;
 using namespace llvm;
 using namespace clang_fuzzer;
 
-char *target_path;
+const char *target_path = nullptr;
 
 void ReportError(llvm::StringRef message) {
   WithColor::error() << message << '\n';
@@ -47,10 +47,24 @@ extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv) {
   signal(SIGPIPE, SIG_IGN);
 #endif
 
-  target_path = ::getenv("LLDB_FUZZER_TARGET");
+  // `target_path` can be set by either the "--lldb_fuzzer_target" commandline
+  // flag or the "LLDB_FUZZER_TARGET" environment variable. Arbitrarily, we
+  // always do flag parsing and only check the environment variable if the
+  // commandline flag is not set.
+  for (int i = 1; i < *argc; ++i) {
+    auto this_arg = llvm::StringRef((*argv)[i]);
+    WithColor::note() << "argv[" << i << "] = " << this_arg << "\n";
+    if (this_arg.consume_front("--lldb_fuzzer_target="))
+      target_path = this_arg.data();
+  }
+
+  if (!target_path)
+    target_path = ::getenv("LLDB_FUZZER_TARGET");
+
   if (!target_path)
-    ReportError(
-        "no target path specified in with the LLDB_FUZZER_TARGET variable");
+    ReportError("No target path specified. Set one either as an environment "
+                "variable (i.e. LLDB_FUZZER_TARGET=target_path) or pass as a "
+                "command line flag (i.e. --lldb_fuzzer_target=target_path).");
 
   if (!sys::fs::exists(target_path))
     ReportError(formatv("target path '{0}' does not exist", target_path).str());