[CMake] Allow parent projects to use in-source builds

LLVM checks if it is performing an in-source build and then stop the
build. However, this check is also triggered if LLVM is being build as
part of a parent project, which prevents the parent project itself from
using in-source builds. For example, CMake allows a parent project to
specify the output of its subproject:

add_subdirectory(llvm llvm_build)

This tells CMake to conduct an out-tree build of LLVM, which without
this patch will still fails because what is being tested is the parent
project, not LLVM. This is fixed by using the "CURRENT" variable, which
is only concerned by the CMakeLists that is actually bein processed at
the moment.

Tests:
Ran `make check-llvm`.

Patch by Henrique Jung <henriquenj_AT_gmail_DOT_com>

llvm-svn: 316142