Added comment in IProblem and filed Bug 509643: [api] Ensure that raw IProblem ID integer literals are unique

Conflicts:
	org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/regression/GenericsRegressionTest_1_8.java
	org.eclipse.jdt.core/compiler/org/eclipse/jdt/core/compiler/IProblem.java
	org.eclipse.jdt.core/compiler/org/eclipse/jdt/internal/compiler/problem/ProblemReporter.java
	org.eclipse.jdt.core/compiler/org/eclipse/jdt/internal/compiler/problem/messages.properties