[NFC] Fix #106873 - update assignment tracking docs (#106959)

Author: OCHyams

llvm/docs/AssignmentTracking.md

@@ -20,12 +20,14 @@ LLVM pass writers, and minimal disruption to LLVM in general.
 
 ## Status and usage
 
-**Status**: Experimental work in progress. Enabling is strongly advised against
-except for development and testing.
+**Status**: Enabled by default in Clang but disabled under some circumstances
+(which can be overridden with the `forced` option, see below). `opt` will not
+run the pass unless asked (`-passes=declare-to-assign`).
 
-**Enable in Clang**: `-Xclang -fexperimental-assignment-tracking`
+**Flag**:
+`-Xclang -fexperimental-assignment-tracking=<disabled|enabled|forced>`
 
-That causes Clang to get LLVM to run the pass `declare-to-assign`. The pass
+When enabled Clang gets LLVM to run the pass `declare-to-assign`. The pass
 converts conventional debug records to assignment tracking metadata and sets
 the module flag `debug-info-assignment-tracking` to the value `i1 true`. To
 check whether assignment tracking is enabled for a module call
@@ -88,7 +90,7 @@ int fun(int a) {
 ```
 compiled without optimisations:
 ```
-$ clang++ test.cpp -o test.ll -emit-llvm -S -g -O0 -Xclang -fexperimental-assignment-tracking
+$ clang++ test.cpp -o test.ll -emit-llvm -S -g -O0 -Xclang -fexperimental-assignment-tracking=enabled
 ```
 we get:
 ```
@@ -194,8 +196,7 @@ the choice at each instruction, iteratively joining the results for each block.
 
 ### TODO list
 
-As this is an experimental work in progress so there are some items we still need
-to tackle:
+Outstanding improvements:
 
 * As mentioned in test llvm/test/DebugInfo/assignment-tracking/X86/diamond-3.ll,
   the analysis should treat escaping calls like untagged stores.
@@ -228,3 +229,8 @@ to tackle:
   that we can only track assignments with fixed offsets and sizes, I think we
   can probably get rid of the address and address-expression part, since it
   will always be computable with the info we have.
+
+* Assignment tracking is disabled by default for LTO and thinLTO builds, and
+  if LLDB debugger tuning has been specified. We should remove these
+  restrictions. See EmitAssemblyHelper::RunOptimizationPipeline in
+  clang/lib/CodeGen/BackendUtil.cpp.