Don't include config.h in public headers

Don't include config.h in public headers

Replace it with llvm-config.h, which defines a subset of
config.h's macros "so that they can be in exported headers
and won't override package specific directives", e.g.,
PACKAGE_NAME.

Endian.h wasn't using any macros at all though, so just delete
the include there instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133712 91177308-0d34-0410-b5e6-96231b3b80d8