Bug#1025058: LLVM_DEFINITIONS is a string and not a list

[Mathieu Malaterre]

Control: forwarded -1
https://reviews.llvm.org/rGd2612ef1c411aba65d7d6e4889261ecec2de52ae

LLVM_DEFINITIONS is a string and not a list

A user may be tempted to just do:

target_compile_definitions(foo PRIVATE ${LLVM_DEFINITIONS})

which would fails. The ${LLVM_DEFINITIONS} expands to a single
argument since the value of the variable is not ;-separated. The
result is that CMake thinks user is trying to specify a single
definition whose name is the entire string value minus a leading -D.