Canonicalize some ENABLE_CLASSIC_FLANG

These CMake variables are used to set python variables in the lit
config. As such, they need to bet set with a valid CMake boolean value
that is also a valid python boolean value (1, True, etc.) and not one
that is not (ON, NO, etc.) to work. This is fragile.

Call the LLVM cmake helper function on these two downstream CMake

Author: RichBarton-Arm

clang/test/CMakeLists.txt

@@ -13,6 +13,7 @@ llvm_canonicalize_cmake_booleans(
   ENABLE_BACKTRACES
   LLVM_BUILD_EXAMPLES
   LLVM_BYE_LINK_INTO_TOOLS
+  LLVM_ENABLE_CLASSIC_FLANG
   LLVM_ENABLE_PLUGINS
   LLVM_ENABLE_ZLIB
   LLVM_ENABLE_ZSTD