[Modules] When things go horribly wrong when reading a module, point at the module cache.

Sometimes people hack on their system headers. In such cases, they'll
need to delete their module cache, but may not know where it is. Add a
note to show them where it is.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181638 91177308-0d34-0410-b5e6-96231b3b80d8
[Modules] Make r180934 more efficient by only loading top-level module maps in system header directories.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181643 91177308-0d34-0410-b5e6-96231b3b80d8
Debug Info: Silently accept template argument packs

We could support the GCC extension DW_TAG_GNU_template_parameter_pack if
we're feeling adventurous, at some point - but I don't think GDB's doing
anything useful with it yet anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181644 91177308-0d34-0410-b5e6-96231b3b80d8