[clang] [Clang][TableGen] Use StringRef in FlattenedSpelling (PR #113809)

[Aaron Ballman via cfe-commits]

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@@ -49,40 +49,38 @@ using namespace llvm;
 namespace {
 
 class FlattenedSpelling {
-  std::string V, N, NS;
+  StringRef V, N, NS;
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AaronBallman wrote:

I'm not keen on this change; it may be correct, but it's pretty dangerous too because this now requires `FlattenedSpelling` to outlive the memory owned by the passed in string references. Keeping a `std::string` here introduces a copy, but that seems reasonable (tablegen is not really performance critical anyway). WDYT?

https://github.com/llvm/llvm-project/pull/113809