Prepare SwiftSyntaxBuilder to be used in Code-Generation

[ahoppen]

• Make all decl, expr, pattern, stmt and type nodes as well as some hand-picked types conform to ExpressibleByStringInterpolation by generating the conformances using gyb
• Move SyntaxExpressibleByStringInterpolation to SwiftSyntaxBuilder
  • I think creating syntax nodes from string using interpolation is mostly useful in contexts where you’re building syntax trees in the SwiftSyntaxBuilder style and thus these functions should live in SwiftSyntaxBuilder instead of SwiftParser. It also reduces the amount of code we have in SwiftParser
• Two SwiftSyntaxBuilder adjustments
  • Fix indentation issue if a parsed syntax node with comments is used inside a builder
  • Add calledExpression label to FunctionCallExpr convenience initializer because otherwise initializing a FunctionCallExpr from a string is ambiguous whether it should be parsed or call the string

[ahoppen]

@DougGregor Could you review the changes in

• Parser.swift
• Patterns.swift
• gyb_syntax_support
• SyntaxExpressibleByStringInterpolationConformances.swift.gyb
• As well as the fact that I’m moving ExpressibleByStringInterpolation to SwiftSyntaxBuilder

@rintaro Could you take a look at

• RawSyntaxNodeProtocol.swift
• RawSyntaxNodes.swift.gyb

[ahoppen]

@swift-ci Please test