[analyzer] NonParamVarRegion should prefer definition over canonical decl

When we construct a `NonParamVarRegion`, we canonicalize the decl to
always use the same entity for consistency.
At the moment that is the canonical decl - which is the first decl in
the redecl chain.

However, this can cause problems with tentative declarations and extern
declarations if we declare an array with unknown bounds.

Consider this C example: https://godbolt.org/z/Kdvr11EqY
```lang=C
typedef typeof(sizeof(int)) size_t;
size_t clang_analyzer_getExtent(const void *p);
void clang_analyzer_dump(size_t n);

extern const unsigned char extern_redecl[];
const unsigned char extern_redecl[] = { 1,2,3,4 };
const unsigned char tentative_redecl[];
const unsigned char tentative_redecl[] = { 1,2,3,4 };

const unsigned char direct_decl[] = { 1,2,3,4 };

void test_redeclaration_extent(void) {
  clang_analyzer_dump(clang_analyzer_getExtent(direct_decl));      // 4
  clang_analyzer_dump(clang_analyzer_getExtent(extern_redecl));    // should be 4 instead of Unknown
  clang_analyzer_dump(clang_analyzer_getExtent(tentative_redecl)); // should be 4 instead of Unknown
}
```

The `getType()` of the canonical decls for the forward declared globals,
will return `IncompleteArrayType`, unlike the
`getDefinition()->getType()`, which would have returned
`ConstantArrayType` of 4 elements.

This makes the `MemRegionManager::getStaticSize()` return `Unknown` as
the extent for the array variables, leading to FNs.

To resolve this, I think we should prefer the definition decl (if
present) over the canonical decl when constructing `NonParamVarRegion`s.

FYI The canonicalization of the decl was introduced by D57619 in 2019.

Differential Revision: https://reviews.llvm.org/D154827