[YAMLParser] Unfold multi-line scalar values (#70898)

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.

llvm/include/llvm/Support/YAMLParser.h

@@ -240,9 +240,14 @@ class ScalarNode final : public Node {
 private:
   StringRef Value;
 
-  StringRef unescapeDoubleQuoted(StringRef UnquotedValue,
-                                 StringRef::size_type Start,
+  StringRef getDoubleQuotedValue(StringRef UnquotedValue,
                                  SmallVectorImpl<char> &Storage) const;
+
+  static StringRef getSingleQuotedValue(StringRef RawValue,
+                                        SmallVectorImpl<char> &Storage);
+
+  static StringRef getPlainValue(StringRef RawValue,
+                                 SmallVectorImpl<char> &Storage);
 };
 
 /// A block scalar node is an opaque datum that can be presented as a