[yaml2obj] - Support the "Offset" key for the .dynsym section.

[grimar]

Summary

Our "implicit" sections are handled separately from regular ones.
It turns out that the "Offset" key is not handled properly for them.

Perhaps we can generalize handling in one place, but before doing that I'd like
to add support and test cases for each implicit section.
(I need this particular single change to unblock another patch that is already on review,
and I guess doing it independently for each section will be cleaner, see below).

In this patch I've removed explicit-dynsym-no-dynstr.yaml to dynsym-section.yaml
and added the new test into. In a follow-up we probably might want
to merge 2 another existent dynsymtab-*.yaml tests into it too.

[jhenderson]

LGTM!

llvm/lib/ObjectYAML/ELFEmitter.cpp

SHeader.sh_info = (RawSec && RawSec->Info) ? (unsigned)(*RawSec->Info)
                                              : findFirstNonGlobal(Symbols) + 1;
   SHeader.sh_entsize = (YAMLSec && YAMLSec->EntSize)
                            ? (uint64_t)(*YAMLSec->EntSize)
                            : sizeof(Elf_Sym);
   SHeader.sh_addralign = YAMLSec ? (uint64_t)YAMLSec->AddressAlign : 8;
 
   assignSectionAddress(SHeader, YAMLSec);
 
-  SHeader.sh_offset = alignToOffset(CBA, SHeader.sh_addralign, /*Offset=*/None);
+  SHeader.sh_offset =
+      alignToOffset(CBA, SHeader.sh_addralign, RawSec ? RawSec->Offset : None);
 
   if (RawSec && (RawSec->Content || RawSec->Size)) {
     assert(Symbols.empty());
     SHeader.sh_size = writeContent(CBA, RawSec->Content, RawSec->Size);
     return;
   }
 
   std::vector<Elf_Sym> Syms =

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

+## Check how we produce SHT_DYNSYM sections.
+
+## Check we do not crash/assert when .dynsym is specified
+## explicitly, but .dynstr is not present.
+## Check we set the sh_link field to 0 for the .dynsym section in this case.
+
+# RUN: yaml2obj %s -o %t1
+# RUN: llvm-readelf --section-headers %t1 | FileCheck %s
+
+# CHECK:      Section Headers:
+# CHECK-NEXT:  [Nr] Name    Type   Address          Off    Size   ES Flg Lk
+# CHECK-NEXT:  [ 0]         NULL   0000000000000000 000000 000000 00 0   0
+# CHECK-NEXT:  [ 1] .dynsym DYNSYM 0000000000000000 000040 000018 18 A   0
+
+--- !ELF
+FileHeader:
+  Class: ELFCLASS64
+  Data:  ELFDATA2LSB
+  Type:  ET_DYN
+Sections:
+  - Name:   .dynsym
+    Type:   SHT_DYNSYM
+    Offset: [[OFFSET=<none>]]
+
+## Check we can use the "Offset" key to to set an arbitrary offset for the SHT_DYNSYM section.
+
+# RUN: yaml2obj %s -DOFFSET=0x100 -o %t2
+# RUN: llvm-readelf --section-headers %t2 | FileCheck %s --check-prefix=OFFSET
+
+# OFFSET:      Section Headers:
+# OFFSET-NEXT:   [Nr] Name    Type   Address          Off    Size   ES Flg Lk Inf Al
+# OFFSET-NEXT:   [ 0]         NULL   0000000000000000 000000 000000 00     0   0  0
+# OFFSET-NEXT:   [ 1] .dynsym DYNSYM 0000000000000000 000100 000018 18   A 0   1  0

llvm/test/tools/yaml2obj/ELF/explicit-dynsym-no-dynstr.yaml

-## Check we do not crash/assert when .dynsym is specified
-## explicitly, but .dynstr is not present.
-
-# RUN: yaml2obj %s -o %t
-# RUN: llvm-readelf --section-headers %t | FileCheck %s
-
-## Check that .dynsym has Link field set to 0.
-
-# CHECK:      Section Headers:
-# CHECK-NEXT:  [Nr] Name    Type   Address          Off    Size   ES Flg Lk
-# CHECK-NEXT:  [ 0]         NULL   0000000000000000 000000 000000 00 0   0
-# CHECK-NEXT:  [ 1] .dynsym DYNSYM 0000000000000000 000040 000018 18 A   0
-
---- !ELF
-FileHeader:
-  Class: ELFCLASS64
-  Data:  ELFDATA2LSB
-  Type:  ET_DYN
-Sections:
-  - Name: .dynsym
-    Type: SHT_DYNSYM