This implements the result of the discussion at:

https://discourse.llvm.org/t/rfc-report-fatal-error-and-the-default-value-of-gencrashdialog/73587

There are two different use cases for report_fatal_error, so replace it
with two functions reportFatalInternalError() and
reportFatalUsageError(). The former indicates a bug in LLVM and
generates a crash dialog. The latter does not. The names have been
suggested by rnk and people seemed to like them.

This replaces a lot of the usages that passed an explicit value for
GenCrashDiag. I did not bulk replace remaining report_fatal_error usage
-- they probably require case by case review for which function to use.

It's barely testable - the test does exercise the code, but wouldn't
fail on an empty implementation. It would cause a memory leak though
(because the error handle wouldn't be unwrapped/reowned) which could be
detected by asan and other leak detectors.

There are some places that want to convert an Error to string, but still
retain the original Error object, for example to emit a non-fatal
warning.

This currently isn't possible, because the entire Error infra is
move-based. And what people end up doing in this case is to move the
Error... twice.

This patch introduces a toStringWithoutConsuming() function to
accommodate this use case. This also requires some infrastructure that
allows visiting Errors without consuming them.

The current API first creates a temporary std::string, then passes it as
a C string, only to then convert it into a std::string for storage, thus
unnecessarily computing the length of the string and copying it.

Move the implementation of the `toString` function from
`llvm/Support/Error.h` to the source file, which allows us to move
`#include "llvm/ADT/StringExtras.h"` to the source file as well.

As `Error.h` is present in a large number of translation units this
means we are unnecessarily bringing in the contents of
`StringExtras.h` - itself a large file with lots of includes - and
slowing down compilation.

Also move the `#include "llvm/ADT/SmallVector.h"` directive to the
source file as it's no longer needed, but this does not give as much of
a benefit.

This reduces the total number of preprocessing tokens across the LLVM
source files in lib from (roughly) 1,920,413,050 to 1,903,629,230 - a
reduction of ~0.87%. This should result in a small improvement in
compilation time.

Differential Revision: https://reviews.llvm.org/D155178

This reverts commit 5be8c89ed6d959d57ae21c168dc4215ce0d05553.

Move the implementation of the `toString` function from
`llvm/Support/Error.h` to the source file, which allows us to move
`#include "llvm/ADT/StringExtras.h"` to the source file as well.

As `Error.h` is present in a large number of translation units this
means we are unnecessarily bringing in the contents of
`StringExtras.h` - itself a large file with lots of includes - and
slowing down compilation.

Also move the `#include "llvm/ADT/SmallVector.h"` directive to the
source file as it's no longer needed, but this does not give as much of
a benefit.

This reduces the total number of preprocessing tokens across the LLVM
source files in lib from (roughly) 1,920,413,050 to 1,903,629,230 - a
reduction of ~0.87%. This should result in a small improvement in
compilation time.

Differential Revision: https://reviews.llvm.org/D155018

This reverts commit 4c0cfd870f78c3f107b23a2bd61504c8ebc22912.

There are still transitive includes missing inside lldb tools.

Move the implementation of the `toString` function from
`llvm/Support/Error.h` to the source file, which allows us to move
`#include "llvm/ADT/StringExtras.h"` to the source file as well.

As `Error.h` is present in a large number of translation units this
means we are unnecessarily bringing in the contents of
`StringExtras.h` - itself a large file with lots of includes - and
slowing down compilation.

Also move the `#include "llvm/ADT/SmallVector.h"` directive to the
source file as it's no longer needed, but this does not give as much of
a benefit.

This reduces the total number of preprocessing tokens across the LLVM
source files in lib from (roughly) 1,920,413,050 to 1,903,629,230 - a
reduction of ~0.87%. This should result in a small improvement in
compilation time.

Differential Revision: https://reviews.llvm.org/D154775

This reverts commit 2e2743b6479c9d90c85a0bf68d83d45c7f9f472d.

More transitive includes of `llvm/ADT/StringExtras.h` need to be
removed.