[analyzer] Drop assertion enforcing that assume args are known constants (#151908)

We sometimes don't know if the operand of [[assume]] is true/false, and
that's okay. We can just ignore the attribute in that case.

If we wanted something more fancy, we could bring the assumption to the
constraints, but dropping them should be just as fine for now.

Fixes #151854

Author: steakhal

clang/lib/StaticAnalyzer/Checkers/AssumeModeling.cpp

@@ -45,7 +45,6 @@ void AssumeModelingChecker::checkPostStmt(const AttributedStmt *A,
       continue;
 
     const auto *Assumption = AssumptionVal.getAsInteger();
-    assert(Assumption && "We should know the exact outcome of an assume expr");
     if (Assumption && Assumption->isZero()) {
       C.addSink();
     }

clang/test/Analysis/cxx23-assume-attribute.cpp

@@ -69,3 +69,8 @@ int assume_and_fallthrough_at_the_same_attrstmt(int a, int b) {
 
   return 0;
 }
+
+void assume_opaque_gh151854_no_crash() {
+  extern bool opaque();
+  [[assume(opaque())]]; // no-crash
+}