clang-cl: Include /FI headers in /showIncludes output.

-H in gcc mode doesn't print -include headers, but they are included in
depfiles written by MMD and friends. Since /showIncludes is what's used instead
of depfiles, printing /FI there seems important (and matches cl.exe).

Instead of giving HeaderIncludeGen more options, just switch on ShowAllHeaders
in clang-cl mode and let clang::InitializePreprocessor() not put -include flags
in the <command line> block. This changes the behavior of -E slightly, and it
removes the <command line> flag from the output triggered by setting the
obscure CC_PRINT_HEADERS=1 env var to true while running clang. Both of these
seem ok to change.

http://reviews.llvm.org/D18401


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264174 91177308-0d34-0410-b5e6-96231b3b80d8