clang-tools-extra/test/clang-tidy/performance-unnecessary-value-param-header.c...

clang-tools-extra/test/clang-tidy/performance-unnecessary-value-param-header.cpp: Resolve flakiness in the test.

Tests would go flaky if;
1. Using %T (not %t)
2. Put a file with common name like header.h into %T
3. Other tests (eg. misc-unused-parameters.cpp) are doing as well

We should avoid using %T unless it really makes sense.

llvm-svn: 307876