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

[modocache]

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".

[beanz]

LGTM!

[modocache]

Great, thanks @beanz! I wanted to make sure I was understanding the terminology :)

CMakeLists.txt

set(CPACK_NSIS_EXTRA_UNINSTALL_COMMANDS
     "ExecWait '$INSTDIR/tools/msbuild/uninstall.bat'")
   if( CMAKE_CL_64 )
     set(CPACK_NSIS_INSTALL_ROOT "$PROGRAMFILES64")
   endif()
 endif()
 include(CPack)
 
 # Sanity check our source directory to make sure that we are not trying to
-# generate an in-tree build (unless on MSVC_IDE, where it is ok), and to make
+# generate an in-source build (unless on MSVC_IDE, where it is ok), and to make
 # sure that we don't have any stray generated files lying around in the tree
 # (which would end up getting picked up by header search, instead of the correct
 # versions).
 if( CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT MSVC_IDE )
   message(FATAL_ERROR "In-source builds are not allowed.
 CMake would overwrite the makefiles distributed with LLVM.
 Please create a directory and run cmake from there, passing the path
 to this source directory as the last argument.