[dsymutil] Fix spurious warnings in MachODebugMapParser (#78794)

When the MachODebugMapParser encounters an object file that cannot be
found on disk, it currently leaves the parser in an incoherent state,
resulting in spurious warnings that can in turn slow down dsymutil.

This fixes #78411.

rdar://117515153

Author: JDevlieghere

llvm/test/tools/dsymutil/ARM/missing-object-warning.test

@@ -0,0 +1,5 @@
+RUN: dsymutil -oso-prepend-path %p/../Inputs --dump-debug-map %p/../Inputs/private/tmp/missing/foobar.out 2>&1 | FileCheck %s
+
+CHECK: bar.o unable to open object file
+CHECK-NOT: could not find object file symbol for symbol _bar
+CHECK-NOT: could not find object file symbol for symbol _main

llvm/test/tools/dsymutil/Inputs/private/tmp/missing/foo.o

@@ -186,6 +186,8 @@ void MachODebugMapParser::addCommonSymbols() {
 /// everything up to add symbols to the new one.
 void MachODebugMapParser::switchToNewDebugMapObject(
     StringRef Filename, sys::TimePoint<std::chrono::seconds> Timestamp) {
+  addCommonSymbols();
+  resetParserState();
 
   SmallString<80> Path(PathPrefix);
   sys::path::append(Path, Filename);
@@ -206,9 +208,6 @@ void MachODebugMapParser::switchToNewDebugMapObject(
     return;
   }
 
-  addCommonSymbols();
-  resetParserState();
-
   CurrentDebugMapObject =
       &Result->addDebugMapObject(Path, Timestamp, MachO::N_OSO);