Teach LLVMConfig to avoid modifying CMAKE_MODULE_PATH

Do not modify this value on the application's behalf and just ensure API
modules are always available next to the LLVMConfig module.  This is
already the case in the install tree so use file(COPY) to make it so in
the build tree.  Include the LLVM-Config API module from next to the
LLVMConfig location.

Contributed by Brad King.
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.
Export lib and exe build target names from install tree

Use the install(TARGETS) command EXPORT option for every library and
executable that we install with LLVM.  Then use the install(EXPORT)
command to provide a "LLVMExports.cmake" file that exports logical
targets for import into applications from our install tree.

The "LLVMExports.cmake" file is not meant for direct inclusion by
application code but should be included by "LLVMConfig.cmake" in a
future change.

Contributed by Brad King.
Export lib and exe build target names from build tree

Record every logical target that we install with install(TARGETS) in a
global LLVM_EXPORTS property.  Then use the export(TARGETS) command to
provide a "LLVMExports.cmake" file that exports logical targets for
import into applications directly from our build tree.

The "LLVMExports.cmake" file is not meant for direct inclusion by
application code but should be included by "LLVMConfig.cmake" in a
future change.

Contributed by Brad King.
Load exported lib and exe targets from LLVMConfig

Teach each package configuration file to load the LLVMExports file for
its corresponding tree.  This will allow application CMake code to use
logical library and executable target names from LLVM as if they were in
our own build process (e.g. LLVMSupport).  CMake will have enough
information to propagate LLVM library link dependencies automatically
while configuring applications.

Contributed by Brad King.