[ELF] - Assign proper values for DefinedSynthetic symbols attached to non-allocatable sections.

[grimar]

Summary

We currently have next issue. In the script from testcase,
non-allocated section is placed before symbol asignment.

Therefore .Sym is attached to non allocatable section. And we calculate it
value wrong. We subtract Body->Section->Addr, but non-allocatable sections
should have zero VA in output and therefore result value is wrong,

We have Body->Section->Addr != 0 for them internally because use it for calculation of size.

Patch fixes calculation of such symbols values.

[grimar]

• Addressed review comments (changes testcase to avoid depending on orphans).

ELF/LinkerScript.cpp

 // symbols whose value is absolute.
 template <class ELFT>
 static void assignSymbol(SymbolAssignment *Cmd, typename ELFT::uint Dot = 0) {
   if (!Cmd->Sym)
     return;
 
   if (auto *Body = dyn_cast<DefinedSynthetic>(Cmd->Sym)) {
     Body->Section = Cmd->Expression.Section();
-    if (Body->Section)
-      Body->Value = Cmd->Expression(Dot) - Body->Section->Addr;
+    if (Body->Section) {
+      uint64_t VA = 0;
+      if (Body->Section->Flags & SHF_ALLOC)
+        VA = Body->Section->Addr;
+      Body->Value = Cmd->Expression(Dot) - VA;
+    }
     return;
   }
 
   cast<DefinedRegular<ELFT>>(Cmd->Sym)->Value = Cmd->Expression(Dot);
 }
 
 template <class ELFT> static void addSymbol(SymbolAssignment *Cmd) {
   if (Cmd->Name == ".")

test/ELF/linkerscript/symbols-and-orphans.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: echo "SECTIONS { . = SIZEOF_HEADERS; \
+# RUN:  .text : { *(.text) }                \
+# RUN:  . = ALIGN(8);                       \
+# RUN:  Sym = .; }" > %t.script
+# RUN: ld.lld -o %t2 --script %t.script %t
+# RUN: llvm-objdump -section-headers -t %t2 | FileCheck %s
+
+# CHECK:      SYMBOL TABLE:
+# CHECK-NEXT: 0000000000000000 *UND* 00000000
+# CHECK-NEXT: 0000000000000160 .strtab 00000000 Sym
+
+.section .orphan,""
+ .quad 0