Add CMAKE_BUILD_TYPE to the list of BOOTSTRAP_DEFAULT_PASSTHROUGH variables

When building clang in stage2, when -DCMAKE_BUILD_TYPE=RelWithDebInfo is set,
the developer can expect that the stage2 clang is built using the same mode.
Especially as the performances are much worst in debug mode.
(Principle of least astonishment)

Differential Revision: https://reviews.llvm.org/D53014