Add support for LLVM12 static linkage

[njamb94]

Hello,

On Linux 22.04, at the moment, it is not possible to statically link LLVM12 due to clangSupport library in LLVMSetup.cmake file. (See make output: link_llvm12_static.log)

From my point of view, the fix lies in modifying LLVMSetup.cmake file to exclude clangSupport library if LLVM version is bellow 15 (like it is done for MSVC):

The choice for llvm static linkage was done by modifying options in CMakeLists.txt:

Question: would it make more sense to make llvm linkage type configurable from the Makefile?
Something like this:

[bkryza]

@njamb94 I'll look into this, I'll have to check how this affects macos and MSVC targets.