[MC] Don't error on numberless .file directives on MachO

[rnk]

Summary

Before r349976, MC ignored such directives when producing an object file
and asserted when re-producing textual assembly output. I turned this
assertion into a hard error in both cases in r349976, but this makes it
unnecessarily difficult to write a single assembly file that supports
both MachO and other object formats that support .file. A user reported
this as PR40578, and we decided to go back to ignoring the directive.

Fixes PR40578

[mstorsjo]

LGTM

llvm/lib/MC/MCParser/AsmParser.cpp

if (Keyword == "md5") {
           parseEscapedString(SourceString))
         return true;
     } else {
       return TokError("unexpected token in '.file' directive");
     }
   }
 
   if (FileNumber == -1) {
-    if (!getContext().getAsmInfo()->hasSingleParameterDotFile())
-      return Error(DirectiveLoc,
-                   "target does not support '.file' without a number");
+    // Ignore the directive if there is no number and the target doesn't support
+    // numberless .file directives. This allows some portability of assembler
+    // between different object file formats.
+    if (getContext().getAsmInfo()->hasSingleParameterDotFile())
       getStreamer().EmitFileDirective(Filename);
   } else {
     // In case there is a -g option as well as debug info from directive .file,
     // we turn off the -g option, directly use the existing debug info instead.
     // Also reset any implicit ".file 0" for the assembler source.
     if (Ctx.getGenDwarfForAssembly()) {
       Ctx.getMCDwarfLineTable(0).resetRootFile();
       Ctx.setGenDwarfForAssembly(false);
     }

llvm/test/MC/MachO/file-single.s

-// RUN: not llvm-mc -triple i386-apple-darwin9 %s -o /dev/null 2>&1 | FileCheck %s
-
-// Previously this crashed MC.
-
-// CHECK: error: target does not support '.file' without a number
-
-        .file "dir/foo"
-        nop

llvm/test/MC/MachO/file.s

 // RUN: llvm-mc -triple i386-apple-darwin9 %s -filetype=obj -o - | llvm-readobj -s -section-data | FileCheck %s
 
+// This number-less file directive is ignored on MachO.
+        .file "bar/baz.s"
+
         .file 1 "dir/foo"
         nop
 
 // CHECK:       Section {
 // CHECK:         Index: 1
 // CHECK-NEXT:    Name: __debug_line
 // CHECK-NEXT:    Segment: __DWARF
 // CHECK-NEXT:    Address: 0x1