Maybe this is more broken than I thought.. from the log:

Checking out LLVM r277204 into 'C:\b\c\b\ClangToTWin\src\third_party\llvm'
Running ['svn', 'checkout', '--force', 'https://llvm.org/svn/llvm-project/llvm/trunk@277204', 'C:\\b\\c\\b\\ClangToTWin\\src\\third_party\\llvm']
D    C:\b\c\b\ClangToTWin\src\third_party\llvm\test\tools\llvm-objdump\malformed-archives.test
[...]
U    C:\b\c\b\ClangToTWin\src\third_party\llvm\include\llvm\DebugInfo\CodeView\ModuleSubstream.h
svn: URL 'https://llvm.org/svn/llvm-project/llvm/trunk/include/llvm/DebugInfo/Msf' of existing directory 'C:\b\c\b\ClangToTWin\src\third_party\llvm\include\llvm\DebugInfo\MSF' does not match expected URL 'https://llvm.org/svn/llvm-project/llvm/trunk/include/llvm/DebugInfo/MSF'
Failed.
Removing C:\b\c\b\ClangToTWin\src\third_party\llvm.
Retrying.
Running ['svn', 'checkout', '--force', 'https://llvm.org/svn/llvm-project/llvm/trunk@277204', 'C:\\b\\c\\b\\ClangToTWin\\src\\third_party\\llvm']
A    C:\b\c\b\ClangToTWin\src\third_party\llvm\.gitignore
[...]
A    C:\b\c\b\ClangToTWin\src\third_party\llvm\lib\DebugInfo\CodeView\SymbolDumper.cpp
A    C:\b\c\b\ClangToTWin\src\third_party\llvm\lib\DebugInfo\MSF
svn: URL 'https://llvm.org/svn/llvm-project/llvm/trunk/lib/DebugInfo/MSF' of existing directory 'C:\b\c\b\ClangToTWin\src\third_party\llvm\lib\DebugInfo\Msf' does not match expected URL 'https://llvm.org/svn/llvm-project/llvm/trunk/lib/DebugInfo/Msf'
Failed.


So we nuke the whole llvm dir, and then "svn co" fails??

Yesh, a plain old "svn co http://llvm.org/svn/llvm-project/llvm/trunk" in a brand new dir fails on my Windows machine:

[..]
A    trunk\include\llvm\DebugInfo\CodeView\MethodListRecordBuilder.h
A    trunk\include\llvm\DebugInfo\CodeView\FieldListRecordBuilder.h
A    trunk\include\llvm\DebugInfo\CodeView\SymbolDumpDelegate.h
A    trunk\include\llvm\DebugInfo\CodeView\TypeRecordBuilder.h
A    trunk\include\llvm\DebugInfo\CodeView\Line.h
A    trunk\include\llvm\DebugInfo\MSF
A    trunk\include\llvm\DebugInfo\MSF\StreamRef.h
A    trunk\include\llvm\DebugInfo\MSF\StreamWriter.h
A    trunk\include\llvm\DebugInfo\MSF\StreamArray.h
A    trunk\include\llvm\DebugInfo\MSF\MSFBuilder.h
A    trunk\include\llvm\DebugInfo\MSF\StreamInterface.h
A    trunk\include\llvm\DebugInfo\MSF\MappedBlockStream.h
A    trunk\include\llvm\DebugInfo\MSF\IMSFFile.h
A    trunk\include\llvm\DebugInfo\MSF\MSFCommon.h
A    trunk\include\llvm\DebugInfo\MSF\StreamReader.h
A    trunk\include\llvm\DebugInfo\MSF\ByteStream.h
A    trunk\include\llvm\DebugInfo\MSF\MSFError.h
A    trunk\include\llvm\DebugInfo\MSF\MSFStreamLayout.h
svn: URL 'http://llvm.org/svn/llvm-project/llvm/trunk/include/llvm/DebugInfo/MSF' of existing directory  trunk\include\llvm\DebugInfo\Msf' does not match expected URL 'http://llvm.org/svn/llvm-project/llvm/trunk/include/llvm/DebugInfo/Msf'

Actually, upstream bots look broken too. From http://lab.llvm.org:8011/builders/clang-x86-win2008-selfhost/builds/9427/steps/svn-llvm/logs/stdio:

Updating '.':
Skipped 'lib\DebugInfo\MSF' -- Node remains in conflict
U    lib\CodeGen\SelectionDAG\LegalizeIntegerTypes.cpp
Skipped 'include\llvm\DebugInfo\MSF' -- Node remains in conflict
U    test\CodeGen\PowerPC\ppc-shrink-wrapping.ll
U    test\CodeGen\Thumb\thumb-shrink-wrapping.ll
A    test\CodeGen\ARM\no_redundant_trunc_for_cmp.ll
U    test\CodeGen\ARM\arm-shrink-wrapping.ll
U    test\CodeGen\AArch64\arm64-shrink-wrapping.ll
Updated to revision 277252.
Summary of conflicts:
  Skipped paths: 2

Aha! Let's see if http://llvm.org/viewvc/llvm-project?rev=277254&view=rev

Yeah, that fixes "svn co" on my Windows machine. Our bots should recover, so I'll mark it fixed.