Remove excessive padding from PTHStatData (NFC)

This diff reorders the fields and removes excessive padding.
This fixes the following warning:

PTHLexer.cpp:629:7: warning: Excessive padding in 'class (anonymous namespace)::PTHStatData' (14 padding bytes, where 6 is optimal). Optimal fields order: Size, ModTime, UniqueID, HasData, IsDirectory, consider reordering the fields or adding explicit padding members.

Patch by: Alexander Shaposhnikov <shal1t712@gmail.com>

Differential Revision: https://reviews.llvm.org/D23826

llvm-svn: 279607