[Assignment Tracking][NFC] Replace LLVM command line option with a module flag

Remove LLVM flag -experimental-assignment-tracking. Assignment tracking is
still enabled from Clang with the command line -Xclang
-fexperimental-assignment-tracking which tells Clang to ask LLVM to run the
pass declare-to-assign. That pass converts conventional debug intrinsics to
assignment tracking metadata. With this patch it now also sets a module flag
debug-info-assignment-tracking with the value `i1 true` (using the flag conflict
rule `Max` since enabling assignment tracking on IR that contains only
conventional debug intrinsics should cause no issues).

Update the docs and tests too.

Reviewed By: CarlosAlbertoEnciso

Differential Revision: https://reviews.llvm.org/D142027

llvm/lib/Transforms/Utils/InlineFunction.cpp

@@ -2302,7 +2302,7 @@ llvm::InlineResult llvm::InlineFunction(CallBase &CB, InlineFunctionInfo &IFI,
     fixupLineNumbers(Caller, FirstNewBlock, &CB,
                      CalledFunc->getSubprogram() != nullptr);
 
-    if (getEnableAssignmentTracking()) {
+    if (isAssignmentTrackingEnabled(*Caller->getParent())) {
       // Interpret inlined stores to caller-local variables as assignments.
       trackInlinedStores(FirstNewBlock, Caller->end(), CB);