[CMake] Explicit bootstrap options override any passthrough ones.

The https://reviews.llvm.org/D53014 added CMAKE_BUILD_TYPE to
the list of BOOTSTRAP_DEFAULT_PASSTHROUGH variables.

The downside is that both stage-1 and stage-2 configurations
are the same. So it is not possible to build different stage
configurations.

This patch allow explicit bootstrap options to override any
passthrough ones.

For instance, the following settings would build:
  stage-1 (Release) and stage-2(Debug)

-DCMAKE_BUILD_TYPE=Release -DBOOTSTRAP_CMAKE_BUILD_TYPE=Debug

Reviewed By: @beanz

Differential Revision: https://reviews.llvm.org/D131755