[NFC] Rename LLVM_NO_DEAD_STRIP

Summary:
The variable LLVM_NO_DEAD_STRIP is set in LLVM cmake files when building executables that might make use of plugins .The name of the variable does not convey the actual intended usage (i.e. for use with tools that have plugins), just what the eventual effect of setting in on some (i.e. not garbage collecting unused symbols).

This patch renames it to LLVM_SUPPORT_PLUGINS to convey the intended usage, which will allow subsequent patches to add behavior to support that in different ways without confusion about whether it will do on, for example, non-gnu platforms.

Reviewers: hubert.reinterpretcast, stevewan

Reviewed By: stevewan

Subscribers: cfe-commits, mgorny, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D69356