[clang] Switch to LLVM_ENABLE_IDE

r344555 switched LLVM to guarding install targets with LLVM_ENABLE_IDE
instead of CMAKE_CONFIGURATION_TYPES, which expresses the intent more
directly and can be overridden by a user. Make the corresponding change
in clang. LLVM_ENABLE_IDE is computed by HandleLLVMOptions, so it should
be available for both standalone and integrated builds.

Differential Revision: https://reviews.llvm.org/D58284

llvm-svn: 354525