[libclang] Revert part of r290025, "Remove the 'extern "C"' blocks from the implementation files."

mingw32-ld complains missing symbols in exports,

  Cannot export clang_findIncludesInFileWithBlock: symbol not defined
  Cannot export clang_findReferencesInFileWithBlock: symbol not defined
  Cannot export clang_visitChildrenWithBlock: symbol not defined

They are excluded conditionally in header along has_blocks.

We should do either;

  1. Exclude also function bodies conditionally, and introduce "optional" exporter.
  2. Give dummy function bodies for them.
  3. Implement functions w/o blocks.

llvm-svn: 290113