[modules] Track how 'header' directives were written in module map files,
rather than trying to extract this information from the FileEntry after the
fact.

This has a number of beneficial effects. For instance, diagnostic messages for
failed module builds give a path relative to the "module root" rather than an
absolute file path, and the contents of the module includes file is no longer
dependent on what files the including TU happened to inspect prior to
triggering the module build.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223095 91177308-0d34-0410-b5e6-96231b3b80d8
Remove special case for aarch64 static vs. PIC code in iOS kernel code.

I added this check a while back but then made a note to myself that it
should be completely unnecessary since iOS always uses PIC code-gen for
aarch64. Since I could never come up with any reason why it would be
necessary, I'm just going to remove it and we'll see if anything breaks.
rdar://problem/13627985

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223097 91177308-0d34-0410-b5e6-96231b3b80d8
Add a test that ensures the Clang driver behaves itself when the PATH
environment variable is changed to strange things out from under it.
Prior to r223099 in LLVM, these test cases would crash in various ways
(assert fails, stack exhaustion, etc.).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223100 91177308-0d34-0410-b5e6-96231b3b80d8