[CMake] Fix typo: "in-tree" -> "in-source" (NFC)

Summary:
*In-source builds* of LLVM, in which a user invokes `cmake` from within the
LLVM source directory, or invokes `cmake -B/path/to/source/dir/of/llvm`,
are explicitly checked for and disallowed by LLVM's `CMakeLists.txt`.

*In-tree builds*, on the other hand, refer to when the source directories
of projects such as Clang are nested within the `llvm/tools` source
directory. These are not disallowed, and are in fact a common way of
building LLVM and Clang.

Revise the comment to match the logic underneath it: it checks for an
"in-source build", not an "in-tree build".

Reviewers: beanz

Reviewed By: beanz

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D38317

llvm-svn: 314348