buildbot failure in LLVM on clang-x64-ninja-win7

[Sanjoy Das via llvm-commits]

I have not been able to reproduce this issue on a Release without
asserts bootstrap on OSX or Linux.  I'm trying a Release+Asserts
bootstrap now.

Since I don't have access to a Windows box, some help on narrowing
down the issue will be much appreciated.

For instance, is it possible to build and test each of the revisions
between r251045 and r251053?  Narrowing down the failure to one
revision will be very helpful.

PS: I noticed the `Force build` form, but wasn't sure if I was okay
using it.

-- Sanjoy

llvm.buildmaster at lab.llvm.org wrote:
 > The Buildbot has detected a new failure on builder 
clang-x64-ninja-win7 while building llvm.
 > Full details are available at:
 >   http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/6404
 >
 > Buildbot URL: http://lab.llvm.org:8011/
 >
 > Buildslave for this Build: windows7-buildbot
 >
 > Build Reason: scheduler
 > Build Source Stamp: [branch trunk] 251053
 > Blamelist: sanjoy
 >
 > BUILD FAILED: failed ninja check 2
 >
 > sincerely,
 >   -The Buildbot
 >
 >
 >