LLVM_DEFINITIONS is a string variable containing a list of arguments
to pass to the compiler. When CMake's add_definitions is passed a
string variable, this is interpreted as one argument. To make it
behave properly, the string variable needs to be split into a list.
Despite the fact that add_definitions isn't supposed to be used like
the LLVM docs recommended, it worked fine in practice in many cases.
If the first argument in LLVM_DEFINITIONS is of the form -DFOO=42
instead of plain -DFOO, the rest of the string is treated as value
to this define. I.e. if LLVM_DEFINITIONS consists of -DFOO=42 -DBAR,
CMake ended up passing -DFOO="42 -DBAR" to the compiler.
for discussion on the matter.
Changing LLVM_DEFINITIONS to be a list variable would possibly be
more disruptive; instead keep the variable defined as before but
change the recommendation for how to use it. Then projects using it
can gradually be updated to follow the new recommendation.