[CodeCompletion][NFC] Extract a function that looks for block decl type locs.

This commit extracts a new function named `findTypeLocationForBlockDecl` 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

llvm-svn: 284467