[clang-tools-extra] [clang-doc] simplify filename selection for namespaces (PR #162885)

[Paul Kirth via cfe-commits]

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@@ -30,6 +30,9 @@ namespace doc {
 // SHA1'd hash of a USR.
 using SymbolID = std::array<uint8_t, 20>;
 
+static const SymbolID GlobalNamespace = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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ilovepi wrote:

```suggestion
constexpr SymbolID GlobalNamespaceID = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
```
I could imagine we may want to use GlobalNamespace as a name/concept.

Alternatively we could just have a helper API `isGlobalNamespace()` that does this check w/o needing a global constant. 

Also, I don't recall what putting `static` on a constant does in a header, but its probably better if it was just  use `constexpr` instead.

https://github.com/llvm/llvm-project/pull/162885