[CMake] Unbreak add_llvm_external_project when external projects are specified.
[CMake] Unbreak add_llvm_external_project when external projects are specified.

LLVM_EXTERNAL_*_SOURCE_DIR is reset as PATH with set(CACHE PATH).
Then the CACHE PATH variable, LLVM_EXTERNAL_*_SOURCE_DIR, is normalized as
${CMAKE_SOURCE_DIR}/${path_var} if ${path_var} is relative.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@242120 91177308-0d34-0410-b5e6-96231b3b80d8

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