Simple LLVM project as smoke test

[ekilmer]

With the recent addition of LLVM 14, there have been a few bugs that prevent expected behavior during our tool builds. These bugs could have been prevented with a simple LLVM project that can be used as a smoke-test to make sure certain features of the LLVM package(s) are behaving as expected.

The simple LLVM project should have the following features (feel free to request more):

• Find and link against LLVM libraries
• Find and link against Clang libraries
• Use the found Clang C and C++ compiler to compile sample source files
• Find and link against MLIR libraries