[CMake] Make LLVMConfig.cmake variable names match in-tree names

With the runtimes build we're trying to use LLVMConfig.cmake as a way of providing LLVM_* variables that are needed to behave as if the project is building in tree. To make this work we need to rename two variables by dropping the "S" from the end. This makes the variables match the in-tree names.

This renames:
LLVM_INCLUDE_DIRS -> LLVM_INCLUDE_DIR
LLVM_LIBRARY_DIRS -> LLVM_LIBRARY_DIR

The versions ending in S are not used in-tree anywhere. This also cleans up LLVM_LIBRARY_DIR being set to the same value with and without the "S".

llvm-svn: 280013