[HLSL] Enable HLSL availability diagnostic on exported library functions and add tests

[hekota]

This is a tracking work item to enable HLSL availability diagnostic on exported library functions and to add tests. This can happen as soon as #92071 is resolved, HLSL functions have internal linkage by default and the export keyword is implemented.

The diagnostic of exported function can be enabled in DiagnoseHLSLAvailability::RunOnTranslationUnit.

The tests should be added to the files in clang\test\SemaHLSL\Availability for all diagnostic modes. For example this is the minimum that should be added to AvailabilityDefaultLibrary.hlsl:

// Exported function without body
export void exportedFunction(float f);

// Exported function with body
export void exportedFunction(float f) {
  // expected-error@#exportedFunction_fx_call {{'fx' is only available on Shader Model 6.5 or newer}}
  // expected-note@#fx {{'fx' has been marked as being introduced in Shader Model 6.5 here, but the deployment target is Shader Model 6.0}}
  float A = fx(f); // #exportedFunction_fx_call

  // API with shader-stage-specific availability in exported library function
  // - no errors expected because the actual shader stage this function
  // will be used in not known at this time
  float B = fy(f);
  float C = fz(f);
}