Execute unimplemented llvm.* intrinsics likewise external functions

[operasfantom]

Summary:

This PR closes #1528.

Added derived class KIntrinsic which inherits from KCallable. This aims to distinguish that from KFunction and create a simple IR call inside ExternalDispatcherImpl::createDispatcher.

Checklist:

• The PR addresses a single issue. If it can be divided into multiple independent PRs, please do so.
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• There are test cases for the code you added or modified OR no such test cases are required.

[operasfantom]

I think I might need a little help with this PR, until then it remains as a draft. The current error I can't deal with is the following:

# command stderr:
/tmp/klee_src/test/Intrinsics/ExternalCallConcrete.ll:28:28: error: CHECK-EXTERNAL-CONCRETE: expected string not found in input
; CHECK-EXTERNAL-CONCRETE: KLEE: done: completed paths = 3
                           ^
<stdin>:8:54: note: scanning from here
KLEE: WARNING ONCE: calling external: llvm.minnum.f32_0(1065353216, 1073741824) at [no debug info]
                                                     ^
<stdin>:15:23: note: possible intended match here
LLVM ERROR: Broken function found, compilation aborted!

Thus, apparently, the external call was executed successfully but then tests were not generated. I couldn't reproduce that LLVM ERROR either on newer or older versions of LLVM in my environment. Does anyone guess what it could be?