Add modules support for lldb headers in include/

Summary:
This patch adds a modulemap which allows compiling the lldb headers into C++ modules
(for example in builds with LLVM_ENABLE_MODULES=On).

Even though most of the affected code has been cleaned up to work with the more strict
C++ module semantics, there are still some workarounds left in the current modulemap
(the most obvious one is the big `lldb` wrapper module).

It also moves the Obj-C++ files in lldb to their own subdirectories. This was necessary
because we need to filter out the modules flags for this code.

Note: With the latest clang and libstdc++ it seems necessary to have a STL C++ module
to get a working LLVM_ENABLE_MODULES build for lldb. Otherwise clang will falsely
detect ODR violations in the textually included STL code inside the lldb modules.

Reviewers: aprantl, bruno

Reviewed By: aprantl, bruno

Subscribers: mgorny, yamaguchi, v.g.vassilev, lldb-commits

Differential Revision: https://reviews.llvm.org/D47929

llvm-svn: 334611