AIX out-of-box memory soft limits are often
insufficient to run LLVM on reasonably size
inputs. Thus, we often encounter users who run
into spurious out of memory errors.

This change raises the memory soft limits
to the hard limits at LLVM startup to prevent
these types of issues.

This AutoConvert.h header frequently gets mislabeled as an unused
include because it is guarded by MVS internally and every usage is also
guarded. This refactors the change to remove this guard and instead make
these functions a noop on other non-z/OS platforms.

Identified with misc-include-cleaner.

This patch renames the functions in AutoConvert.h/cpp to have a less
generic name because they are z/OS specific.

Previously, some tools such as `clang` or `lld` which require strict
order for certain command-line options, such as `clang -cc1` or `lld
-flavor`, would not longer work on Windows, when these tools were linked
as part of `llvm-driver`. This was caused by `InitLLVM` which was part
of the `*_main()` function of these tools, which in turn calls
`windows::GetCommandLineArguments`. That function completly replaces
argc/argv by new UTF-8 contents, so any ajustements to argc/argv made by
`llvm-driver` prior to calling these tools was reset.

`InitLLVM` is now called by the `llvm-driver`. Any tool that
participates in (or is part of) the `llvm-driver` doesn't call
`InitLLVM` anymore.

This patch completes the support for EBCDIC I/O support on z/OS using the autoconversion functions.

The cleanup was manual, but assisted by "include-what-you-use". It consists in

1. Removing unused forward declaration. No impact expected.
2. Removing unused headers in .cpp files. No impact expected.
3. Removing unused headers in .h files. This removes implicit dependencies and
   is generally considered a good thing, but this may break downstream builds.
   I've updated llvm, clang, lld, lldb and mlir deps, and included a list of the
   modification in the second part of the commit.
4. Replacing header inclusion by forward declaration. This has the same impact
   as 3.

Notable changes:

- llvm/Support/TargetParser.h no longer includes llvm/Support/AArch64TargetParser.h nor llvm/Support/ARMTargetParser.h
- llvm/Support/TypeSize.h no longer includes llvm/Support/WithColor.h
- llvm/Support/YAMLTraits.h no longer includes llvm/Support/Regex.h
- llvm/ADT/SmallVector.h no longer includes llvm/Support/MemAlloc.h nor llvm/Support/ErrorHandling.h

You may need to add some of these headers in your compilation units, if needs be.

As an hint to the impact of the cleanup, running

clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/Support/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l

before: 8000919 lines
after:  7917500 lines

Reduced dependencies also helps incremental rebuilds and is more ccache
friendly, something not shown by the above metric :-)

Discourse thread on the topic: https://llvm.discourse.group/t/include-what-you-use-include-cleanup/5831

Following the discussion in D112753, this moves the HTTPClient from Support to Debuginfod library so that tools depending on Support do not automatically depend on Curl as well. This also removes `HTTPClient::initialize()` and `HTTPClient::cleanup()` from `InitLLVM` so these steps should be implemented by user tools instead.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D115131