[flang] [llvm] [llvm][flang] Silence warning, resume -Werror builds of flang (PR #175053)

[Peter Klausler via llvm-commits]

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@@ -211,6 +211,8 @@ class SmallVectorTemplateCommon
       this->assertSafeToReferenceAfterResize(From, 0);
       this->assertSafeToReferenceAfterResize(To - 1, 0);
     }
+    (void)From;
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klausler wrote:

The `(void)` technique is used in the following function for the same purpose, so I chose consistency.

https://github.com/llvm/llvm-project/pull/175053