[yaml2obj] - Add support of Offset for .strtab/.shstrtab/.dynstr sections.

[grimar]

Summary

These sections are implicit and handled a bit differently.
Currently the "Offset" is ignored for them.
This patch fixes an issue.

[jhenderson]

LGTM.

llvm/lib/ObjectYAML/ELFEmitter.cpp

void ELFState<ELFT>::initStrtabSectionHeader(Elf_Shdr &SHeader, StringRef Name,
   zero(SHeader);
   SHeader.sh_name = getSectionNameOffset(Name);
   SHeader.sh_type = YAMLSec ? YAMLSec->Type : ELF::SHT_STRTAB;
   SHeader.sh_addralign = YAMLSec ? (uint64_t)YAMLSec->AddressAlign : 1;
 
   ELFYAML::RawContentSection *RawSec =
       dyn_cast_or_null<ELFYAML::RawContentSection>(YAMLSec);
 
-  SHeader.sh_offset = alignToOffset(CBA, SHeader.sh_addralign, /*Offset=*/None);
+  SHeader.sh_offset = alignToOffset(CBA, SHeader.sh_addralign,
+                                    YAMLSec ? YAMLSec->Offset : None);
 
   if (RawSec && (RawSec->Content || RawSec->Size)) {
     SHeader.sh_size = writeContent(CBA, RawSec->Content, RawSec->Size);
   } else {
     if (raw_ostream *OS = CBA.getRawOS(STB.getSize()))
       STB.write(*OS);
     SHeader.sh_size = STB.getSize();
   }

llvm/test/tools/yaml2obj/ELF/strtab-implicit-sections.yaml

 FileHeader:
   Class: ELFCLASS64
   Data:  ELFDATA2LSB
   Type:  ET_DYN
 Sections:
   - Name: .strtab
     Type: SHT_STRTAB
     Flags: [[STRTABFLAGS=<none>]]
+    Offset: [[STRTABOFFSET=<none>]]
   - Name: .shstrtab
     Type: SHT_STRTAB
     Flags: [[SHSTRTABFLAGS=<none>]]
+    Offset: [[SHSTRTABOFFSET=<none>]]
   - Name: .dynstr
     Type: SHT_STRTAB
     Flags: [[DYNSTRFLAGS=<none>]]
+    Offset: [[DYNSTROFFSET=<none>]]
 
-## Check we can set arbitrary flags for .strtab/.dynstr.
+## Check we can set arbitrary flags and offsets for
+## implicit string table sections.
 
 # RUN: yaml2obj --docnum=1 \
-# RUN:   -DSTRTABFLAGS="[ SHF_ALLOC, SHF_STRINGS ]" \
-# RUN:     -DSHSTRTABFLAGS="[ SHF_ALLOC, SHF_WRITE ]" \
-# RUN:       -DDYNSTRFLAGS="[ SHF_STRINGS ]" %s -o %t2
+# RUN:   -DSTRTABFLAGS="[ SHF_ALLOC, SHF_STRINGS ]" -DSTRTABOFFSET=0x100 \
+# RUN:     -DSHSTRTABFLAGS="[ SHF_ALLOC, SHF_WRITE ]" -DSHSTRTABOFFSET=0x200 \
+# RUN:       -DDYNSTRFLAGS="[ SHF_STRINGS ]" -DDYNSTROFFSET=0x300 %s -o %t2
 # RUN: llvm-readelf %t2 --sections | FileCheck %s --check-prefix=CASE2
 
-#      CASE2: [Nr] Name      Type   {{.*}} Flg Lk
-#      CASE2: [ 1] .strtab   STRTAB {{.*}}  AS 0 
-# CASE2-NEXT: [ 2] .shstrtab STRTAB {{.*}}  WA 0 
-# CASE2-NEXT: [ 3] .dynstr   STRTAB {{.*}}   S 0 
+#      CASE2: [Nr] Name      Type   Address          Off    {{.*}} Flg Lk
+#      CASE2: [ 1] .strtab   STRTAB 0000000000000000 000100 {{.*}}  AS 0 
+# CASE2-NEXT: [ 2] .shstrtab STRTAB 0000000000000001 000200 {{.*}}  WA 0 
+# CASE2-NEXT: [ 3] .dynstr   STRTAB 0000000000000000 000300 {{.*}}   S 0 
 
 ## Check no flags are set by default for implicit string table sections
 ## when they are not described in the YAML.
 
 # RUN: yaml2obj --docnum=2 %s -o %t3
 # RUN: llvm-readelf %t3 --sections | FileCheck %s --check-prefix=CASE3
 
 #      CASE3:   [Nr] Name      Type   {{.*}} ES Flg Lk