[Caching] Use swift-frontend as exectuable name in replays

Don't try to figure out the executable names during replay from
libSwiftScan.dylib. The actual executable path for the process actually
doesn't matter in this case to reconstruct the invocation and might
actually be misleading.

Just use `swift-frontend` as a placeheader executable name for
in-process cache replay.

rdar://132758308

Author: cachemeifyoucan

test/CAS/swift-scan-diagnostics.swift

@@ -54,6 +54,7 @@
 // PARSEABLE-NEXT: {
 // PARSEABLE-NEXT:   "kind": "began",
 // PARSEABLE-NEXT:   "name": "compile",
+// PRASEABLE-NEXT:   "command": "swift-frontend -frontend
 
 // PARSEABLE:   "kind": "finished",
 // PARSEABLE-NEXT:   "name": "compile",

tools/libSwiftScan/SwiftCaching.cpp

@@ -276,16 +276,8 @@ computeCacheKey(llvm::cas::ObjectStore &CAS, llvm::ArrayRef<const char *> Args,
   swift::CompilerInvocation Invocation;
   swift::SourceManager SourceMgr;
   swift::DiagnosticEngine Diags(SourceMgr);
-  llvm::SmallString<128> workingDirectory;
-  llvm::sys::fs::current_path(workingDirectory);
-  llvm::SmallVector<std::unique_ptr<llvm::MemoryBuffer>, 4>
-      configurationFileBuffers;
 
-  std::string MainExecutablePath = llvm::sys::fs::getMainExecutable(
-      "swift-frontend", (void *)swiftscan_cache_replay_compilation);
-
-  if (Invocation.parseArgs(Args, Diags, &configurationFileBuffers,
-                           workingDirectory, MainExecutablePath))
+  if (Invocation.parseArgs(Args, Diags, nullptr, {}, "swift-frontend"))
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "Argument parsing failed");
 
@@ -826,11 +818,10 @@ swiftscan_cache_replay_instance_create(int argc, const char **argv,
   swift::DiagnosticEngine DE(SrcMgr);
   DE.addConsumer(Diags);
 
-  std::string MainExecutablePath = llvm::sys::fs::getMainExecutable(
-      "swift-frontend", (void *)swiftscan_cache_replay_compilation);
-
+  // This is inside a dylib so just use `swift-frontend` as executable path
+  // since the actual executable path is not useful here.
   if (Instance->Invocation.parseArgs(Args, DE, nullptr, {},
-                                     MainExecutablePath)) {
+                                     "swift-frontend")) {
     delete Instance;
     *error = swift::c_string_utils::create_clone(err_msg.c_str());
     return nullptr;