Add new LLVM_EXTERNAL_PROJECTS option to cmake

The new option makes it possible to build external projects as part of
the llvm build without copying (or symlinking) then into llvm/tool with
specifying a few additional cmake variables.

Example usage (2 additional project called foo and bar):
-DLLVM_EXTERNAL_PROJECTS="Foo;Bar"
-DLLVM_EXTERNAL_FOO_SOURCE_DIR=/src/foo
-DLLVM_EXTERNAL_BAR_SOURCE_DIR=/src/bar

Note: This is the extension of the approach we already support for
clang/lldb/poly with adding an option to specify additional supported
projects.

Differential revision: http://reviews.llvm.org/D20838

llvm-svn: 271440