[dexter-tests] Add attribute optnone to main function

The test case optnone-simple-functions.cpp is expected to be compiled
unoptimized even under -O2 because of attribute optnone. But this attribute is
missed for main function, and caused the fail in
https://lab.llvm.org/buildbot/#/builders/217/builds/14046 when it triggered
optimizations. This patch fixes that by adding attribute optnone to main
function.

Differential Revision: https://reviews.llvm.org/D136778

cross-project-tests/debuginfo-tests/dexter-tests/optnone-simple-functions.cpp

@@ -90,6 +90,7 @@ int test5(int test5_val) {
 // DexExpectWatchValue('test5_val', 7, from_line=ref('test5_start'), to_line=ref('test5_end'))
 // DexExpectWatchValue('c', 1, 5, from_line=ref('test5_start'), to_line=ref('test5_end'))
 
+__attribute__((optnone))
 int main() {
   int main_result = 0;
   // DexLabel('main_start')