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

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.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D114717

llvm/lib/MC/MCParser/ELFAsmParser.cpp

@@ -676,14 +676,14 @@ EndStmt:
       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()));

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

@@ -4,6 +4,7 @@
 
 # CHECK: .section        .foo,"aM",@progbits,1
 # CHECK: .section        .rodata.cst8,"aM",@progbits,8
+# CHECK: .section        .lds_bss,"w",@nobits
 
 .section .foo,"aM",@progbits,1
 
@@ -15,3 +16,7 @@
 .section .rodata.cst8,"aM",@progbits,8
 
 .section .rodata.cst8
+
+# Likewise for Type changes
+.section        .lds_bss,"w",@nobits
+.section        .lds_bss