Enforce rule that `export`/`extern` (non cpp) must be `const` (#7113)

* Enforce rule that `export`/`extern` (non cpp) must be `const`

fixes: #5570

Problem:
1. we allow non-const-link-time-var to be linked to a const-link-time-var.
2. problem is that: module use site has const var, so, we emit OpStore %Ptr %Const in IR, this is expected, this is good. We fail because we in reality have a OpStore %Ptr %Var (fails since we need a OpLoad in-between) in IR since the module with our link-time-variable-value is a regular variable.
3. We loose the float_litteral talked about inside the github issue since, we technically don't use our variable "VAL" (we never OpLoad from it), so spirv-opt removes the float_litteral, this is a byproduct of the actual issue.

Solution:
* `export`/`extern` variables must always be `const`. This excludes `__extern_cpp` since `cpp` does not exhibit this issue and works differently.

* format code

* changel logic and tests to only ensure `static const` with `export`/`extern`

* changing the rules: only reqirement is that if we have const we must have static

* remove a spirrious change made

* fix merge

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Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Yong He <yonghe@outlook.com>

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