[CMake] Add LLVM_ENABLE_IDE option to better process sources for IDE's

Summary:
Currently LLVM has no way to support configuring for IDE's like CLion. Like XCode and MSVC's IDE, CLion needs to see all of the headers and tablegen files in order to properly parse the sources.

This patch adds an `LLVM_ENABLE_IDE` option which can be used to configure for IDE's in general. It is used by `LLVMProcessSources.cmake` to determine if the extra source files should be added to the target.

Unfortunately because of the low level of `LLVMProcessSources.cmake`, I'm not sure where the `LLVM_ENABLE_IDE` option should live. I choose `HandleLLVMOptions.cmake` so that out-of-tree Clang builds would correctly configure the option by default.



Reviewers: beanz, mgorny, lebedev.ri

Reviewed By: beanz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40219

llvm-svn: 322349