Rearrange LongVector Execution Tests infrastructure (microsoft#7739)

This change rearranges the long vector execution test infrastructure to
be more static, relying on explicit dispatch tables rather than class
hierarchies, variants and std:::functions.

This change tries to avoid moving code around more than it has to.
Follow up NFC changes will reorganize this to move more code out of the
header.

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Co-authored-by: Ashley Coleman <[email protected]>

Author: damyanp

tools/clang/unittests/HLSLExec/LongVectorTestData.h

@@ -2,8 +2,10 @@
 #define LONGVECTORTESTDATA_H
 
 #include <Verify.h>
+
 #include <limits>
 #include <map>
+#include <ostream>
 #include <string>
 #include <vector>