For better support of some specific GNU extensions some extra
transformation of AST nodes were introduced. These transformations are
very hard to handle. The code is improved in handling of these
extensions by using captured expressions construct.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264709 91177308-0d34-0410-b5e6-96231b3b80d8