This fixes a problem that was exposed by the recently added ItaniumDemangleTest.cpp unittest.
The problem is that Demangle/Compiler.h checks LLVM_ENABLE_DUMP to see how it should define LLVM_DUMP_METHOD, but it doesn't include llvm-config.h so LLVM_ENABLE_DUMP is always undefined. If you have a build that defines LLVM_ENABLE_DUMP and NDEBUG, then Demangle/Compiler.h and Support/Compiler.h will define LLVM_DUMP_METHOD differently.
You can see this by configuring this way:
cmake -DLLVM_ENABLE_DUMP=true -DCMAKE_BUILD_TYPE="Release" -DLLVM_ENABLE_WERROR=true ../llvm
It isn't clear to me whether or not including llvm-config.h introduces a layering issue, hence the review for a one line change.