[CMake] Make specifying invalid build type a fatal error (#72021)

This patch makes it so that specifying an invalid value for
CMAKE_BUILD_TYPE is a fatal error. Having this simply as a warning has
caused me (and probably others) a decent amount of headache. The check
was present before, but was proposed to be modified to a warning in
https://github.com/llvm/llvm-project/issues/60975 and changed to a
warning in c75dbeda. This patch
reenables that behavior to hopefully reduce frustration for people
building LLVM in the common case while still allowing for alternative
build types to be setup without needing to perform source modification
through the addition of a CMake flag.