The `bridging` header for C++ interop was calculating an include
directory using `CMAKE_BINARY_DIR` and `CMAKE_CFG_INTDIR` while the rest
of the headers consistently use `CMAKE_CURRENT_BINARY_DIR` and no
`CMAKE_CFG_INTDIR`.
In some configurations of CMake (for example when using Swift as an
external project of LLVM and building an unified toolchain), this means
that the `brigding` header will end up in a different directory than the
rest of the headers, which complicates testing.
The changes in this commit reuses `SWIFT_INCLUDE_DIR` to keep
consistency with the rest of the headers. Any build started by
`build-script` should not notice the difference.
