COFF: replace a magic number and assert more

Assert that the size of the MD5 result is the same size as the signature
field being populated.  Use the sizeof operator to determine the size of
the field being written rather than hardcoding it to the magic number
16.  NFC.

llvm-svn: 290764

lld/COFF/Writer.cpp

@@ -828,7 +828,10 @@ void Writer::writeBuildId() {
 
   assert(BuildId->DI->Signature.CVSignature == OMF::Signature::PDB70 &&
          "only PDB 7.0 is supported");
-  memcpy(BuildId->DI->PDB70.Signature, Res, 16);
+  assert(sizeof(Res) == sizeof(BuildId->DI->PDB70.Signature) &&
+         "signature size mismatch");
+  memcpy(BuildId->DI->PDB70.Signature, Res,
+         sizeof(codeview::PDB70DebugInfo::Signature));
   // TODO(compnerd) track the Age
   BuildId->DI->PDB70.Age = 1;
 }