Identified with modernize-use-using.

In Unix/DynamicLibrary.inc, it was already known that Cygwin required
use of `RTLD_DEFAULT` as the `Handle` parameter to `DLSym` to search all
modules for a symbol. Unfortunately, RTLD_DEFAULT is defined as NULL, so
the existing checks of the `Process` handle meant `DLSym` would never be
called on Cygwin. Use the existing `&Invalid` sentinel instead of
`nullptr` for the `Process` handle.

Previously, it was possible to load dynamic libraries which would be unloaded on llvm_shutdown(), but recently ManagedStatic removal changed this so that loaded libraries really can't ever be unloaded. This functionality was very useful, and so to add it back in a more explicit way, I've added new getLibrary() and closeLibrary() methods to allow callers to use the very convenient platform independent abstraction that LLVM has for dynamic libraries.

As a specific use case, the onnx-mlir project was using this functionality with an API that allows instancing LLVM so you can compile a shared library, and then load that library, and eventually close the instance (and library) and compile something else. This change to llvm_shutdown causes libraries to leak and also locks the libraries for the entire duration of the program which prevents reusing library names.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D134763

v2:
- fix Windows build issues

Differential Revision: https://reviews.llvm.org/D129127

This reverts commit 38817af6a7271aee0581c56eaea44b8c4cf5ecc7.

Buildbots report a Windows build error. Revert until I can look at it
more carefully.

Differential Revision: https://reviews.llvm.org/D129127

Identified with modernize-use-default-member-init.

This also includes a few cleanup from Support.

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121331

This patch has not been reviewed and was commited by accident.

This reverts commit 788a5d4afe6407e647454a9832a7b4a27fba06bf.