This reverts commit ee232506b870ce5282cc4da5ca493d41d361feb3.
As discussed in https://reviews.llvm.org/D146668 we'll find another way
forward.

The high level goal of this change is to remove lldbTarget's dependency
on lldbPluginProcessUtility. The reason for this existing dependency is
so that we can create the appropriate UnixSignals object based on an
ArchSpec. Instead of using the ArchSpec, we can instead take advantage
of the Platform associated with the current Target.

This is accomplished by adding a new method to Platform,
CreateUnixSignals, which will create the correct UnixSignals object for
us. We then can use `Platform::GetUnixSignals` and rely on that to give
us the correct signals as needed.

Differential Revision: https://reviews.llvm.org/D146263

By adding signal codes to UnixSignals and adding a new function
where you can get a string with optional address and bounds.

Added signal codes to the Linux, FreeBSD and NetBSD signal sets.
I've checked the numbers against the relevant sources.

Each signal code has a code number, description and printing options.
By default you just get the descripton, you can opt into adding either
a fault address or bounds information.

Bounds signals we'll use the description, unless we have the bounds
values in which case we say whether it is an upper or lower bound
issue.

GetCrashReasonString remains in CrashReason because we need it to
be compiled only for platforms with siginfo_t. Ideally it would
move into NativeProcessProtocol, but that is also used
by NativeRegisterContextWindows, where there would be no siginfo_t.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D146044

MIPS Linux support was removed in ce03a862372a6f36d2fcf80dc80052aa155fcae8

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to clean up the "using" declarations, #include
"llvm/ADT/Optional.h", etc.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch adds #include <optional> to those files containing
llvm::Optional<...> or Optional<...>.

I'll post a separate patch to actually replace llvm::Optional with
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

std::optional::value() has undesired exception checking semantics and is
unavailable in some older Xcode. The call sites block std::optional migration.

This patch replaces x.hasValue() with x where x is contextually
convertible to bool.

This reverts commit aa8feeefd3ac6c78ee8f67bf033976fc7d68bc6d.