ExecutionEngine: support IMAGE_REL_AMD64_SECTION relocations

This relocation type is often used for debug information on Windows.  We
would previously abort due to the unreachable for the unhandled
relocation type.  Add support for this to prevent LLDB from aborting if
it encounters this relocation type.

llvm/lib/ExecutionEngine/RuntimeDyld/Targets/RuntimeDyldCOFFX86_64.h

@@ -134,6 +134,13 @@ class RuntimeDyldCOFFX86_64 : public RuntimeDyldCOFF {
       break;
     }
 
+    case COFF::IMAGE_REL_AMD64_SECTION: {
+      assert(static_cast<int16_t>(RE.SectionID) <= INT16_MAX && "Relocation overflow");
+      assert(static_cast<int16_t>(RE.SectionID) >= INT16_MIN && "Relocation underflow");
+      writeBytesUnaligned(RE.SectionID, Target, 2);
+      break;
+    }
+
     default:
       llvm_unreachable("Relocation type not implemented yet!");
       break;

llvm/test/ExecutionEngine/RuntimeDyld/X86/COFF_x86_64.s

@@ -4,6 +4,15 @@
 # RUN:   -dummy-extern external_data=0x2 -verify -check=%s %t/COFF_x86_64.o
 
 
+	.section section,"rx"
+section:
+	.long 0
+Lreloc:
+	.long 0
+# rtdyld-check: *{2}Lreloc = 1
+	.reloc 4, secidx, section+4
+
+
         .text
 	.def	 F;
 	.scl	2;