Trying to fix undefined symbol error caused by iterator variable

[ShashwathiNavada]

When a mapper is declared with an iterator variable inside the map clause, it results in unintended behavior due to the iterator being implicitly created but left uninitialized.
Testcase:

typedef struct myvec{
    size_t len;
    double *data;
} myvec_t;

#pragma omp declare mapper(id:myvec_t v) map( iterator( iterator_variable=0:v.len), tofrom: v.data[iterator_variable]) 

int main() 
{ 
    int errors = 0;
   myvec_t s;
   #pragma omp target map(mapper(id), to:s)
     {
     }
  return 0;
}

The error we get while compiling this is:

/usr/lib64/gcc/x86_64-suse-linux/14/../../../../x86_64-suse-linux/bin/ld: /tmp/test-f70647.o: in function `.omp_mapper._ZTS5myvec.id':
test.cpp:(.text+0x21a): undefined reference to `iterator_variable'
/llvm-project/install/bin/clang-linker-wrapper: error: 'ld' failed
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

This patch tries to fix this by initializing the iterator variable to a null constant.

[chandraghale]

Add LIT test . Also elaborate the description with a test case.