Add the ability to test AOT compilation in CodegenTestBase.

This change adds the ability to test AOT compilation. It also
changes the way that IR is obtained from the compiler by using
LLVMCompiler hooks.

CodegenTestBase is also modified in the following ways:
* New subclass for LLVM IR testing, with existing CompileAndVerifyIr
  moved into the subclass.
* CreateNewModuleWithEmbeddedIR() is removed.
* Calls to a FileCheck test now have a flag about which version
of the IR to use.
* FileCheck functionality is moved into its library.

PiperOrigin-RevId: 166515069