[CodeCompletion][NFC] Extract a function that formats block placeholders.
[CodeCompletion][NFC] Extract a function that formats block placeholders.

This commit extracts a new function named `formatBlockPlaceholder` from
the function `FormatFunctionParameter` so that it can be reused in follow-up
commits that improve code completion for block property setters.

Differential Revision: https://reviews.llvm.org/D25519

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@284468 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit f051915ca117aebec67f145c86f86b10b5e89c33)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang