SPIR-V validator failure with SPV_INTEL_function_pointers extension

The attached bitcode, featuring an indirect call, does translate to SPIR-V but validator complains following:
```
$ llvm-as-15 llvm-spirv-bug-15.ll -o - \
| llvm-spirv-15 --spirv-max-version=1.1 --spirv-ext=+all - -o - \
| spirv-val -
error: line 62: ID '4[%_ZL6add123i]' has not been defined
%_ZL6add123i_0 = OpConstantFunctionPointerINTEL %_ptr_Function_3 %_ZL6add123i
```
Perhaps, there is a missing forward declaration in the produced SPIR-V binary.

[llvm-spirv-bug-15.zip](https://github.com/KhronosGroup/SPIRV-LLVM-Translator/files/11684840/llvm-spirv-bug-15.zip)

Start with the attached llvm-spirv-bug-15.zip and reproduce the llvm-as-15, llvm-spirv-15, and spirv-val pipeline shown in the report. Inspect the generated SPIR-V around OpConstantFunctionPointerINTEL and determine what is needed for spirv-val to accept the indirect-call output; done means the reproduced translation validates successfully.

Written by the indexing model from the issue text.