Implement MSVC header search algorithm in MicrosoftMode.

Follows algorithm described here: http://msdn.microsoft.com/en-us/library/36k2cdd4.aspx

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198082 91177308-0d34-0410-b5e6-96231b3b80d8