clang-format: Revamp function declaration/definition indentation.

clang-format: Revamp function declaration/definition indentation.

Key changes:
- Correctly (well ...) distinguish function declarations and variable
  declarations with ()-initialization.
- Don't indent when breaking function declarations/definitions after the
  return type.
- Indent variable declarations and typedefs when breaking after the
  type.

This fixes llvm.org/PR17999.

llvm-svn: 212591