Revert "Replace trivial use of external rc.exe by writing our own .res file."

This reverts commit d4c7e9fc63c10dbab0c30186ef8575474a704496.

This is done in order to address the failure of CrWinClangLLD etc. bots.
These throw an error of "side-by-side configuration is incorrect" during
compilation, which sounds suspiciously related to these manifest
changes.

Revert "Switch external cvtres.exe for llvm's own resource library."

This reverts commit 71fe8ef283a9dab9a3f21432c98466cbc23990d1.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306618 91177308-0d34-0410-b5e6-96231b3b80d8

lib/BinaryFormat/Magic.cpp

@@ -51,8 +51,7 @@ file_magic llvm::identify_magic(StringRef Magic) {
       return file_magic::coff_import_library;
     }
     // Windows resource file
-    if (Magic.size() >= sizeof(COFF::WinResMagic) &&
-        memcmp(Magic.data(), COFF::WinResMagic, sizeof(COFF::WinResMagic)) == 0)
+    if (startswith(Magic, "\0\0\0\0\x20\0\0\0\xFF"))
       return file_magic::windows_resource;
     // 0x0000 = COFF unknown machine type
     if (Magic[1] == 0)