gdb/testsuite: fix gdb.mi/mi-var-cp.exp with clang

gdb/testsuite: fix gdb.mi/mi-var-cp.exp with clang

The inline tests in gdb.mi/mi-var-cp.cc were failing when using clang to
run the test. This happened because inline tests want to step past the C
statements and then run the TCL tests, but in mi-var-cp.cc the statement
to be stepped past is "return s2.i;". Since clang links the epilogue
information to the return statement, not the closing brace,
single-stepping past return had us exiting the function - which made the
expressions invalid.

This commit fixes this by making the function have 2 C statements, and
the return one be after all inline tests, so we know GDB won't leave the
function before running the create_varobj tests.

Approved-By: Tom Tromey <tom@tromey.com>