Implement IMAGE_REL_AMD64_SECREL for RuntimeDyldCOFFX86_64

[lanza]

Summary

lldb on Windows uses the ExecutionEngine for expression evaluation
and hits the llvm_unreachable due to this relocation. Thus, implement
the relocation and add a test to verify it's function.

[lanza]

clang format so Saleem doesn't murder me

[lanza]

ping @compnerd

[lanza]

landed in https://reviews.llvm.org/rL348904

lib/ExecutionEngine/RuntimeDyld/Targets/RuntimeDyldCOFFX86_64.h

case COFF::IMAGE_REL_AMD64_ADDR32NB: {
       break;
     }
 
     case COFF::IMAGE_REL_AMD64_ADDR64: {
       writeBytesUnaligned(Value + RE.Addend, Target, 8);
       break;
     }
 
+    case COFF::IMAGE_REL_AMD64_SECREL: {
+      assert(static_cast<int64_t>(RE.Addend) <= INT32_MAX && "Relocation overflow");
+      assert(static_cast<int64_t>(RE.Addend) >= INT32_MIN && "Relocation underflow");
+      writeBytesUnaligned(RE.Addend, Target, 4);
+      break;
+    }
+
     default:
       llvm_unreachable("Relocation type not implemented yet!");
       break;
     }
   }
 
   std::tuple<uint64_t, uint64_t, uint64_t>
   generateRelocationStub(unsigned SectionID, StringRef TargetName,

test/ExecutionEngine/RuntimeDyld/X86/COFF_x86_64_IMGREL.s

 
 F:                                      # @F
 # rtdyld-check: decode_operand(inst1, 3) = section_addr(COFF_x86_64_IMGREL.o, .text)+0-40960000000000
 inst1:
     mov %ebx, F@IMGREL
 # rtdyld-check: decode_operand(inst2, 3) = section_addr(COFF_x86_64_IMGREL.o, .rdata)[phone]
 inst2:
     mov %ebx, (__constdata@imgrel+5)
+        .data
+        .space 375
+rel1:
+# rtdyld-check: *{4}rel1 = string - section_addr(COFF_x86_64_IMGREL.o, .data)
+  .secrel32 string
+
+# We explicitly add padding to put string outside of the 16bit address space
+# (absolute and as an offset from .data), so that relocations involving
+# 32bit addresses / offsets are not accidentally truncated to 16 bits.
+        .space 65536
+        .global string
+        .align 1
+string:
+        .asciz "Hello World\n"