[dsymutil] Don't print swift interface timestamp warning if we have no timestamp

[JDevlieghere]

Summary

If we have no timestamp, don't print the corresponding warning. The binary holder already successfully ignores these cases, but the warning for swift interface files was lacking it.

rdar://86036385

[aprantl]

llvm/tools/dsymutil/DwarfLinkerForBinary.cpp

417–418

We could have a local variable called epochStart or something to make this more readable, but wither way is fine.

llvm/test/tools/dsymutil/X86/swift-ast-x86_64.test

 RUN: dsymutil -oso-prepend-path %p/.. %p/../Inputs/swift-ast.macho.x86_64 -o %T/swift-ast.dSYM -verbose -no-swiftmodule-timestamp | FileCheck %s --check-prefix=DSYMUTIL
+RUN: dsymutil -oso-prepend-path %p/.. %p/../Inputs/swift-ast.macho.x86_64 -o %T/swift-ast.dSYM -verbose | FileCheck %s --check-prefix=DSYMUTIL
 RUN: llvm-readobj --sections --section-data %T/swift-ast.dSYM/Contents/Resources/DWARF/swift-ast.macho.x86_64 | FileCheck %s --check-prefix=READOBJ
 RUN: llvm-dwarfdump --show-section-sizes %T/swift-ast.dSYM/Contents/Resources/DWARF/swift-ast.macho.x86_64 | FileCheck %s --check-prefix=DWARFDUMP
 
 The tested object file has been created by the dummy Swift code:
 let x = 1
 
 Compiled with:
   swiftc /tmp/test.swift -Onone -target x86_64-apple-macosx10.9 -c
   ld swift-ast.o -add_ast_path Inputs/swift-ast.swiftmodule -arch x86_64 -lSystem -macosx_version_min 10.9.0
 
 DSYMUTIL: filename:{{.*}}swift-ast.swiftmodule
 DSYMUTIL-NOT: The file was not recognized as a valid object file
+DSYMUTIL-NOT: timestamp mismatch
 DSYMUTIL: DEBUG MAP OBJECT:{{.*}}swift-ast.swiftmodule
 
 READOBJ: Name:{{.*}}__swift_ast
 READOBJ: |SWIFTMODULE DATA|
 READOBJ-NEXT: |.|
 
 DWARFDUMP: __swift_ast
 
-RUN: dsymutil -oso-prepend-path %p/.. %p/../Inputs/swift-ast.macho.x86_64 -no-output -verbose 2>&1 | FileCheck %s --check-prefix=TIMESTAMP
-TIMESTAMP: warning: {{.*}}/swift-ast.swiftmodule: timestamp mismatch between swift interface file ({{.*}}) and debug map ({{.*}})
-
 RUN: dsymutil -s %T/swift-ast.dSYM/Contents/Resources/DWARF/swift-ast.macho.x86_64 | FileCheck %s --check-prefix=NAST
 NAST-NOT: N_AST

llvm/tools/dsymutil/DwarfLinkerForBinary.cpp

if (Obj->getType() == MachO::N_AST) {
         continue;
       }
       if (!Options.NoTimestamp) {
         // The modification can have sub-second precision so we need to cast
         // away the extra precision that's not present in the debug map.
         auto ModificationTime =
             std::chrono::time_point_cast<std::chrono::seconds>(
                 Stat.getLastModificationTime());
-        if (ModificationTime != Obj->getTimestamp()) {
+        if (Obj->getTimestamp() != sys::TimePoint<>() &&
+            ModificationTime != Obj->getTimestamp()) {
           // Not using the helper here as we can easily stream TimePoint<>.
           WithColor::warning()
               << File << ": timestamp mismatch between swift interface file ("
               << sys::TimePoint<>(ModificationTime) << ") and debug map ("
               << sys::TimePoint<>(Obj->getTimestamp()) << ")\n";
           continue;
         }
       }