Add a block for C clients in the compatibility header. This block
contains only the `@cdecl` functions that are printed using only C
types.
This C block is printed above the Objective-C and C++ blocks as if we
add support for `@cdecl` types other languages should be able to
reference them in function signatures. Other languages block don't
duplicate printing the `@cdecl` functions either as they are already
accessible to them.
The frontend option '-clang-header-expose-module' allows the user to specify that APIs from an imported module have been exposed in another generated header, and thus APIs that depend on them can be safely exposed in the current generated header.
This change extends the clang header printer to start emitting C++ classes for Swift struct types with the correct struct layout in them (size + alignment)
