[cmake] Export gtest/gtest_main and its dependencies via a special build tree only...

[cmake] Export gtest/gtest_main and its dependencies via a special build tree only cmake exports file.

Previously, gtest/gtest_main were not exported via cmake. The intention here was
to ensure that users whom are linking against the LLVM install tree would not
get the gtest/gtest_main targets. This prevents downstream projects that link
against the LLVM build tree (i.e. Swift) from getting this dependency
information in their cmake builds. Without such dependency information, linker
issues can result on linux due to LLVMSupport being put before gtest on the
linker command line.

This commit preserves behavior that we want for the install tree, while adding
support for the build tree by:

1. The special casing for gtest/gtest_main in the add_llvm_library code is
removed in favor of a flag called "BUILDTREE_ONLY". If this is set, then the
library is communicating that it is only meant to be exported into the build
tree and is not meant to be installed or exported via the install tree. This
part is just a tweak to remove the special case, the underlying code is the
same.

2. The cmake code that exports cmake targets for the build tree has special code
to import an additional targets file called
LLVMBuildTreeOnlyExports.cmake. Additionally the extra targets are added to the
LLVMConfig.cmake's LLVM_EXPORTED_TARGETS variable. In contrast, the
"installation" cmake file uses the normal LLVM_EXPORTS_TARGETS as before and
does not include the extra exports file. This is implemented by
defining/undefining variables when performing a configure of the build/install
tree LLVMConfig.cmake files.

llvm-svn: 281085