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

clang/CMakeLists.txt

@@ -706,6 +706,7 @@ if (CLANG_ENABLE_BOOTSTRAP)
     CMAKE_CXX_COMPILER_LAUNCHER
     CMAKE_MAKE_PROGRAM
     CMAKE_OSX_ARCHITECTURES
+    CMAKE_BUILD_TYPE
     LLVM_ENABLE_PROJECTS
     LLVM_ENABLE_RUNTIMES)