Give files from #line the characteristics of the current file

This allows #line directives to appear in system headers that have code
that clang would normally warn on. This is compatible with GCC, which is
easy to test by running `gcc -E`.

Fixes PR30752

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@303582 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed