Add CMAKE_BUILD_TYPE to the list of BOOTSTRAP_DEFAULT_PASSTHROUGH variables

[sylvestre.ledru]

Summary

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)

[serge-sans-paille]

LGTM, sorry for the late review.

[sylvestre.ledru]

Rebase of the patch

clang/CMakeLists.txt

set(_BOOTSTRAP_DEFAULT_PASSTHROUGH
     CLANG_VENDOR
     LLVM_VERSION_SUFFIX
     LLVM_BINUTILS_INCDIR
     CLANG_REPOSITORY_STRING
     CMAKE_C_COMPILER_LAUNCHER
     CMAKE_CXX_COMPILER_LAUNCHER
     CMAKE_MAKE_PROGRAM
     CMAKE_OSX_ARCHITECTURES
+    CMAKE_BUILD_TYPE
     LLVM_ENABLE_PROJECTS
     LLVM_ENABLE_RUNTIMES)
 
   # We don't need to depend on compiler-rt/libcxx if we're building instrumented
   # because the next stage will use the same compiler used to build this stage.
   if(NOT LLVM_BUILD_INSTRUMENTED)
     if(TARGET compiler-rt)
       add_dependencies(clang-bootstrap-deps compiler-rt)