CodeGen: ConstStructBuilder must verify packed constraints after padding

This reverts commit r220169 which reverted r220153.  However, it also
contains additional changes:
- We may need to add padding *after* we've packed the struct.  This
  occurs when the aligned next field offset is greater than the new
  field's offset.  When this occurs, we make the struct packed.
  *However*, once packed the next field offset might be less than the
  new feild's offset.  It is in this case that we might further pad the
  struct.
- We would pad structs which were perfectly sized!  This behavior is
  immensely old.  This behavior came from blindly subtracting
  NextFieldOffsetInChars from RecordSize.  This doesn't take into
  account the fact that the struct might have a greater overall
  alignment than the last field.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220175 91177308-0d34-0410-b5e6-96231b3b80d8
[modules] Add support for #include_next.

#include_next interacts poorly with modules: it depends on where in the list of
include paths the current file was found. Files covered by module maps are not
found in include search paths when building the module (and are not found in
include search paths when @importing the module either), so this isn't really
meaningful. Instead, we fake up the result that #include_next *should* have
given: find the first path that would have resulted in the given file being
picked, and search from there onwards.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220177 91177308-0d34-0410-b5e6-96231b3b80d8