[MC][ELF] Fix accepting abbreviated form with Type change

[burnus]

Summary

Follow up to D92052 and D94072, exposed due to D107707

Many assemblers to permit that only the first .section contains all
the attributes like '.lds_bss,"w",@nobits' and later section only
use the name ('.lds_bss') inheriting those attributes from the first
section. I turned out that the case that Type changed was missed
when implementing it - and D107707 make it much more likely to hit
that issue. That's fixed by this commit.

[MaskRay]

LGTM.

llvm/lib/MC/MCParser/ELFAsmParser.cpp

if (const MCSectionELF *Section =
         Flags |= ELF::SHF_GROUP;
       }
   }
 
   MCSectionELF *Section =
       getContext().getELFSection(SectionName, Type, Flags, Size, GroupName,
                                  IsComdat, UniqueID, LinkedToSym);
   getStreamer().SwitchSection(Section, Subsection);
-  if (Section->getType() != Type &&
+  // Check that flags are used consistently. However, the GNU assembler permits
+  // to leave out in subsequent uses of the same sections; for compatibility,
+  // do likewise.
+  if (!TypeName.empty() && Section->getType() != Type &&
       !allowSectionTypeMismatch(getContext().getTargetTriple(), SectionName,
                                 Type))
     Error(loc, "changed section type for " + SectionName + ", expected: 0x" +
                    utohexstr(Section->getType()));
-  // Check that flags are used consistently. However, the GNU assembler permits
-  // to leave out in subsequent uses of the same sections; for compatibility,
-  // do likewise.
   if ((extraFlags || Size || !TypeName.empty()) && Section->getFlags() != Flags)
     Error(loc, "changed section flags for " + SectionName + ", expected: 0x" +
                    utohexstr(Section->getFlags()));
   if ((extraFlags || Size || !TypeName.empty()) &&
       Section->getEntrySize() != Size)
     Error(loc, "changed section entsize for " + SectionName +
                    ", expected: " + Twine(Section->getEntrySize()));
 

llvm/test/MC/ELF/section-omitted-attributes.s

 # RUN: llvm-mc -triple=x86_64 %s -o - | FileCheck %s
 
 # If section flags and other attributes are omitted, don't error.
 
 # CHECK: .section        .foo,"aM",@progbits,1
 # CHECK: .section        .rodata.cst8,"aM",@progbits,8
+# CHECK: .section        .lds_bss,"w",@nobits
 
 .section .foo,"aM",@progbits,1
 
 .section .foo
 
 .pushsection .foo
 
 # Likewise, except that the '.rodata' prefix implies SHF_ALLOC.
 .section .rodata.cst8,"aM",@progbits,8
 
 .section .rodata.cst8
+
+# Likewise for Type changes
+.section        .lds_bss,"w",@nobits
+.section        .lds_bss