[dump] Make LLVM_ENABLE_DUMP independent, and move to llvm-config.h

Summary: Make LLVM_ENABLE_DUMP independent LLVM_ENABLE_ASSERTIONS,
move it to llvm-config.h, and update description.

Differential Revision: https://reviews.llvm.org/D38406

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320111 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed