Silence warning about unnecessary lambda captures with clang

The `unused-lambda-capture` will be triggered by the following code:

```cpp
void foo(int);
int main() {
    const int i = 0;
    auto l = [i](){foo(i);};
}
```

The issue here is that reading from the constant variable `i` does not
constitute an ODR-use, and consequently the variable does not have to be
captured. See
https://github.com/llvm/llvm-project/issues/34213#issuecomment-980987311
for a related discussion.

However, MSVC sees it differently.

In order to make both compilers happy, mark this variable with
`AZ_UNUSED`, since lambda captures can't be marked with attributes like
`[[maybe_unused]]`.

Signed-off-by: Chris Burel <burelc@amazon.com>

Code/Tools/TestImpactFramework/Runtime/Code/Source/Artifact/Factory/TestImpactTestRunSuiteFactory.cpp

@@ -69,6 +69,7 @@ namespace TestImpact
                     const auto getDuration = [Keys](const AZ::rapidxml::xml_node<>* node)
                     AZ_POP_DISABLE_WARNING
                     {
+                        AZ_UNUSED(Keys); // Clang reports a warning that capturing Keys is not necessary because it is not odr-used
                         const AZStd::string duration = node->first_attribute(Keys[DurationKey])->value();
                         return AZStd::chrono::milliseconds(static_cast<AZStd::sys_time_t>(AZStd::stof(duration) * 1000.f));
                     };
@@ -86,6 +87,7 @@ namespace TestImpact
                         const auto getStatus = [Keys](const AZ::rapidxml::xml_node<>* node)
                         AZ_POP_DISABLE_WARNING
                         {
+                            AZ_UNUSED(Keys); // Clang reports a warning that capturing Keys is not necessary because it is not odr-used
                             const AZStd::string status = node->first_attribute(Keys[StatusKey])->value();
                             if (status == Keys[RunKey])
                             {