[yaml2obj] Fix bug when referencing items in SectionHeaderTable

[jhenderson]

Summary

There was an off-by-one error caused by an index (which included an index for the null section header) being used to check against the size of a list of sections (which didn't include the null section header).

This is a partial fix for https://bugs.llvm.org/show_bug.cgi?id=50506.

[MaskRay]

Looks great!

llvm/lib/ObjectYAML/ELFEmitter.cpp

if (SectionHeaders.IsImplicit ||
       (SectionHeaders.NoHeaders && !SectionHeaders.NoHeaders.getValue()) ||
       SectionHeaders.isDefault())
     return Index;
 
   assert(!SectionHeaders.NoHeaders.getValueOr(false) ||
          !SectionHeaders.Sections);
   size_t FirstExcluded =
       SectionHeaders.Sections ? SectionHeaders.Sections->size() : 0;
-  if (Index >= FirstExcluded) {
+  if (Index > FirstExcluded) {
     if (LocSym.empty())
       reportError("unable to link '" + LocSec + "' to excluded section '" + S +
                   "'");
     else
       reportError("excluded section referenced: '" + S + "'  by symbol '" +
                   LocSym + "'");
   }
   return Index;

llvm/test/tools/yaml2obj/ELF/section-headers.yaml

FileHeader:
   Class: ELFCLASS64
   Data:  ELFDATA2LSB
   Type:  ET_REL
 Sections:
   - Type: SectionHeaderTable
     Sections:
       - Name: .strtab
       - Name: .shstrtab
+
+## Show that we can reference both the first and last section in the section
+## header table, via sh_link.
+
+# RUN: yaml2obj %s --docnum=14 -o %t14
+# RUN: llvm-readelf --sections %t14 | FileCheck %s --check-prefix=LINK
+
+--- !ELF
+FileHeader:
+  Class: ELFCLASS64
+  Data:  ELFDATA2LSB
+  Type:  ET_REL
+Sections:
+  - Name: .strtab
+    Type: SHT_STRTAB
+  - Type: SectionHeaderTable
+    Sections:
+      - Name: .strtab
+      - Name: .ref.first
+      - Name: .ref.last
+      - Name: .shstrtab
+  - Name: .ref.first
+    Type: SHT_PROGBITS
+    Link: .strtab
+  - Name: .ref.last
+    Type: SHT_PROGBITS
+    Link: .shstrtab
+  - Name: .shstrtab
+    Type: SHT_STRTAB
+
+# LINK:      There are 5 section headers
+# LINK:        [Nr] Name       Type     Address  Off      Size     ES       Flg Lk
+# LINK:        [ 1] .strtab
+# LINK-NEXT:   [ 2] .ref.first PROGBITS [[#%x,]] [[#%x,]] [[#%x,]] [[#%x,]]      1
+# LINK-NEXT:   [ 3] .ref.last  PROGBITS [[#%x,]] [[#%x,]] [[#%x,]] [[#%x,]]      4
+# LINK-NEXT:   [ 4] .shstrtab