[llvm][cmake] Use $<CONFIG> instead of ${CMAKE_CFG_INTDIR} for llvm-config (#75417)

${CMAKE_CFG_INTDIR} does not work correctly for llvm-config when
building with the Ninja Multi-Config generator. It tries to find files
in the ${CONFIGURATION} directory. Using the $<CONFIG> generator
expression instead fixes this.

Really this needs to be done everywhere as ${CMAKE_CFG_INTDIR} is
deprecated as of 3.21 (LLVM's current min version is 3.20), but this is
sufficient to get `check-llvm` to pass.

See https://cmake.org/cmake/help/latest/variable/CMAKE_CFG_INTDIR.html