Preprocessor: preserve whitespace in -traditional-cpp mode.

Note that unlike GNU cpp we currently do not preserve whitespace in macros
(even in -traditional-cpp mode).

<rdar://problem/12897179>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175778 91177308-0d34-0410-b5e6-96231b3b80d8