Mangle the IsSystem bit into the .pcm file name

Mangle the IsSystem bit into the .pcm file name

When mangling the module map path into a .pcm file name, also mangle the
IsSystem bit, which can also depend on the header search paths. For
example, the user may change from -I to -isystem.  This can affect
diagnostics in the importing TU.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228966 91177308-0d34-0410-b5e6-96231b3b80d8