[clang] [Clang] Don't build static analyzer if disabled (PR #212024)

[Nico Weber via cfe-commits]

nico wrote:

More ideas:

* Pull just the static analyzer into its own dylib, this will likely not affect runtime, and then clang could not depend on it and clang-tidy could (might have to allow undefined symbols during link time in case clang-cpp.so and clang-sa-cpp.so have circular deps)
* Check if clang-tools-extra is in LLVM_ENABLE_PROJECTS and land this patch with that additional check (i.e. do this patch, but only if clang-tools-extra isn't enabled). A bit janky, but maybe enough.

(Out of interest, what's the benefit of the shared build if all code goes into a single dylib? Just disk size?)

https://github.com/llvm/llvm-project/pull/212024