[llvm] [opt] Do not exit on first emitError (PR #194858)

[Matt Arsenault via llvm-commits]

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@@ -31,6 +32,15 @@ class TargetMachine;
 class ToolOutputFile;
 class TargetLibraryInfoImpl;
 
+/// A custom diagnostic handler for opt that consumes diagnostics so that the
+/// default LLVMContext::diagnose() does not call exit() on the first error.
+/// opt can then drain remaining diagnostics from a single pass run and report
+/// them all, mirroring llc's behavior so that opt remains a faithful test
+/// driver for passes that report errors via LLVMContext::emitError.
+struct OptDiagnosticHandler : public DiagnosticHandler {
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arsenm wrote:

This should probably just be the default handler 

https://github.com/llvm/llvm-project/pull/194858