[clang] [clang][modules] Shrink the size of `Module::Headers` (PR #113395)

[via cfe-commits]

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@@ -271,8 +271,22 @@ class alignas(8) Module {
     DirectoryEntryRef Entry;
   };
 
-  /// The headers that are part of this module.
-  SmallVector<Header, 2> Headers[5];
+private:
+  unsigned HeaderKindBeginIndex[6] = {};
+  SmallVector<Header, 2> HeadersStorage;
+
+public:
+  ArrayRef<Header> getHeaders(HeaderKind HK) const {
+    auto BeginIt = HeadersStorage.begin() + HeaderKindBeginIndex[HK];
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cor3ntin wrote:

I think we should assert that `HK is < NumHeaderKinds`

https://github.com/llvm/llvm-project/pull/113395