Fix formatting of lambda introducers with initializers.

Most of the work was already done when we introduced a look-behind based
lambda introducer detection.

This patch finishes the transition by completely relying on the simple
lambda introducer detection and simply recursing into normal
brace-parsing code to parse until the end of the introducer.

This fixes initializers in lambdas, including nested lambdas.

Before:
  auto a = [b = [c = 42]{}]{};
  auto b = [c = &i + 23]{};

After:
  auto a = [b = [c = 42] {}] {};
  auto b = [c = &i + 23] {};

llvm-svn: 313622