Reverting r246497 (which requires also reverting r246524 and r246521 to avoid merge conflicts). It broke the build on MSVC 2015. It also broke an MSVC 2013 bot with testing issues.

llvm\tools\clang\lib\serialization\MultiOnDiskHashTable.h(117):
error C2065: 'Files': undeclared identifier

http://bb.pgr.jp/builders/ninja-clang-i686-msc18-R/builds/2917

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246546 91177308-0d34-0410-b5e6-96231b3b80d8