[llvm] [cmake] Provide split include paths in LLVMConfig

Modify LLVMConfig to provide split variables for in-source and generated
include paths.  Currently, it uses a single value for both
LLVM_INCLUDE_DIRS and LLVM_INCLUDE_DIR which works for install tree but
fails hard at build tree (where LLVM_INCLUDE_DIR incorrectly contains
multiple values).

Instead, put the generated directory in LLVM_INCLUDE_DIR, and the source
tree in LLVM_MAIN_INCLUDE_DIR which is consistent with in-LLVM builds.
For install tree, both variables will have the same value.

Differential Revision: https://reviews.llvm.org/D58109

llvm-svn: 353924