Add a convenience initializer for TernaryExpr

[fwcd]

Generating ternary expressions using SwiftSyntaxBuilder with correct formatting is quite verbose currently:

TernaryExpr(
  conditionExpression: ...,
  questionMark: .infixQuestionMarkToken(leadingTrivia: .space, trailingTrivia: .space),
  firstChoice: ...,
  colonMark: .colonToken(leadingTrivia: .space),
  secondChoice: ...
)

This PR adds a convenience initializer that makes constructed TernaryExprs in Swift almost as terse as their generated equivalents:

TernaryExpr(
  if: ...,
  then: ...,
  else: ...
)

I've used the if-then-else terminology for the labels since it is short, clear and easily disambiguable from the generated initializer, but feel free to suggest alternative spellings.

[fwcd]

@swift-ci please test