This patch is part of a series that adds origin-tracking to the debugify
source location coverage checks, allowing us to report symbolized stack
traces of the point where missing source locations appear.

This patch adds a pair of new functions in `signals.h` that can be used
to collect and symbolize stack traces respectively. This has major
implementation overlap with the existing stack trace
collection/symbolizing methods, but the existing functions are
specialized for dumping a stack trace to stderr when LLVM crashes, while
these new functions are meant to be called repeatedly during the
execution of the program, and therefore we need a separate set of
functions.

startswith/endswith wrap starts_with/ends_with and will eventually go away (to more closely match string_view)

When the environment LLVM_ENABLE_SYMBOLIZER_MARKUP is set, if
llvm-symbolizer fails or is disabled, this change will print a backtrace
in llvm-symbolizer markup instead of falling back to in-process
symbolization mechanisms.

This allows llvm-symbolizer to be run on the output later to produce a
high quality backtrace, even for fully-stripped LLVM utilities.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D139750

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
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

Interestingly, MathExtras.h doesn't use <cmath> declaration, so move it out of
that header and include it when needed.

No functional change intended, but there's no longer a transitive include
fromMathExtras.h to cmath.

In order to allow compiling with -Werror=global-constructors with c++20 and above.

Discussion: https://discourse.llvm.org/t/llvm-lib-support-signals-cpp-fails-to-compile-due-to-werror-global-constructors/61070

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D122067

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 moves the dependency of several files on include/llvm/ADT/STLExtras.h to
the much shorter llvm/ADT/STLArrayExtras.h

Differential Revision: https://reviews.llvm.org/D118342