[clang] [clang] Constant-evaluate format strings as last resort (PR #135864)

[via cfe-commits]

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@@ -3,6 +3,11 @@
 // RUN: %clang_cc1 -fblocks -fsyntax-only -verify -Wformat-nonliteral -isystem %S/Inputs -triple=x86_64-unknown-fuchsia %s
 // RUN: %clang_cc1 -fblocks -fsyntax-only -verify -Wformat-nonliteral -isystem %S/Inputs -triple=x86_64-linux-android %s
 
+// expected-note at -5{{format string was constant-evaluated}}
+// ^^^ there will be a <scratch space> SourceLocation caused by the
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cor3ntin wrote:

Like @shafik and @tbaederr - I have a slight concern with performance.
In general, we should avoid checking format strings when these diagnostics are not enabled. That would at least lead to less work in system headers. 

Benchmarking sounds like a good idea.
But I don't have a better solution than using a scratch space. 

https://github.com/llvm/llvm-project/pull/135864