Revert "Make naming in Host.h in line with coding standards."

Revert "Make naming in Host.h in line with coding standards."

This reverts r299062, which caused build failures on Windows.
It also reverts the attempts to fix the windows builds in r299064 and r299065.
The introduction of namespace llvm::sys::detail makes MSVC, and seemingly also
mingw, complain about ambiguity with the existing namespace llvm::detail.
E.g.:
C:\b\slave\sanitizer-windows\llvm\include\llvm/Support/MathExtras.h(184): error C2872: 'detail': ambiguous symbol
C:\b\slave\sanitizer-windows\llvm\include\llvm/Support/PointerLikeTypeTraits.h(31): note: could be 'llvm::detail'
C:\b\slave\sanitizer-windows\llvm\include\llvm/Support/Host.h(80): note: or       'llvm::sys::detail'

In r299064 and r299065 I tried to fix these ambiguities, based on the errors
reported in the log files. It seems however that the build stops early when
this kind of error is encountered, and many build-then-fix-iterations on
Windows may be needed to fix this. Therefore reverting r299062 for now to
get the build working again on Windows.

llvm-svn: 299066