In C++17 and later, "return {A, B};" guarantees copy elision for a
std::pair return type, ensuring the object is constructed directly in
the return slot.  This patch updates those instances under Support/.

It is almost always simpler to use {} instead of std::nullopt to
initialize an empty ArrayRef. This patch changes all occurrences I could
find in LLVM itself. In future the ArrayRef(std::nullopt_t) constructor
could be deprecated or removed.

Long scalar values can be split into multiple lines to improve
readability. The rules are described in Section 6.5. "Line Folding",
https://yaml.org/spec/1.2.2/#65-line-folding. In addition, for flow
scalar styles, the Spec states that "All leading and trailing white
space characters on each line are excluded from the content",
https://yaml.org/spec/1.2.2/#73-flow-scalar-styles.

The patch implements these unfolding rules for double-quoted,
single-quoted, and plain scalars.

Leading white spaces on the line following an escaped line break should
be excluded from the content.
See https://yaml.org/spec/1.2.2/#731-double-quoted-style.

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

The `YAMLParser.h` header file claims support for YAML 1.2 with a few
deviations, but our plain scalar parsing failed to parse some valid YAML
according to the spec. This change puts us more in compliance with the
YAML spec, now letting us parse plain scalars containing additional
special characters in cases where they are not ambiguous.

In some cases plain scalars are currently parsed with a trailing
newline. In particular this shows up often when parsing JSON files, e.g.
note the `\n` after `456` below:
```
$ cat test.yaml
{
  "foo": 123,
  "bar": 456
}
$ yaml-bench test.yaml -canonical
%YAML 1.2
---
!!map {
  ? !!str "foo"
  : !!str "123",
  ? !!str "bar"
  : !!str "456\n",
}
...
```
The trailing whitespace ends up causing the conversion of the scalar to
int/bool/etc. to fail, causing the issue seen here:
https://github.com/llvm/llvm-project/issues/15877

From reading the YAML spec (https://yaml.org/spec/1.2.2/#733-plain-style)
it seems like plain scalars should never end with whitespace, so this
change trims all trailing whitespace characters from the
value (specifically `b-line-feed`, `b-carriage-return`, `s-space`, and
`s-tab`).

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D137118

These source files no longer use None, so they do not need to include
None.h.

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