Add "has issued errors" method on a compilation (#1091)

`Compilation::getSemanticDiagnostics()` caches its result and is meant
to be called on a finished compilation for which no more diagnostics
will be issued.

For use cases where user code using slang as a library wants to issue
its own diagnostics, but doesn't want to deal with an AST which failed
elaboration, provide a `hasIssuedErrors()` method to find out early
if the elaboration failed.

include/slang/ast/Compilation.h

@@ -708,6 +708,9 @@ class SLANG_EXPORT Compilation : public BumpAllocator {
     /// Creates an empty ImplicitTypeSyntax object.
     const syntax::ImplicitTypeSyntax& createEmptyTypeSyntax(SourceLocation loc);
 
+    /// Queries if any errors have been issued on any scope within this compilation.
+    bool hasIssuedErrors() const { return numErrors > 0; };
+
     /// @{
 
 private: