[clang] [Clang][NFC] Clarify some SourceManager related code (PR #153527)

[Mariya Podchishchaeva via cfe-commits]

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@@ -1095,6 +1095,8 @@ prepareAndFilterRanges(const SmallVectorImpl<CharSourceRange> &Ranges,
 
     unsigned StartColumn = SM.getExpansionColumnNumber(Begin);
     unsigned EndColumn = SM.getExpansionColumnNumber(End);
+    assert(StartColumn && "StartColumn has a value of 0");
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Fznamznon wrote:

Can we maybe explain why it can't be 0?

https://github.com/llvm/llvm-project/pull/153527