[Ada] Detection of illegal constituent assignments
This patch modifies the analysis of assignment statements to detect an illegal
attempt to alter the value of single protected type Part_Of constituent when
inside a protected function.
* sem_ch5.adb (Analyze_Assignment): Assignments to variables that act
as Part_Of consituents of single protected types are illegal when they
take place inside a protected function.
(Diagnose_Non_Variable_Lhs): Use Within_Function to check for an
enclosing function.
(Is_Protected_Part_Of_Constituent): New routine.
(Within_Function): New routine.
gcc/testsuite/
* gnat.dg/protected_func.adb, gnat.dg/protected_func.ads: New testcase.