Provide LLVMConfig in both build and install tree

Create separate package configuration files "LLVMConfig.cmake" for the
LLVM build and install trees so that each can have information specific
to its tree.  Configure each with the corresponding include, lib, and
cmake directories.  Include the "LLVM-Config" API modules directly from
the configured cmake modules directory.

In the install tree, compute the installation prefix relative to the
file location.  In the build tree, provide information specific to the
build tree for use by tools like Clang that can build externally against
the LLVM build tree.  Prefix such values in "LLVM_BUILD_" and comment
them as such.

Contributed by Brad King.

llvm-svn: 201048