Replace FIXME with TODO in toolchain code (#4506)

toolchain/check/testdata/class/fail_abstract.carbon

@@ -126,7 +126,7 @@ class Derived {
 }
 
 fn Return(a: Abstract) -> Abstract {
-  // FIXME: Seems like this would be better off failing with "function returns abstract type" here instead of this \/
+  // TODO: Seems like this would be better off failing with "function returns abstract type" here instead of this \/
   // CHECK:STDERR: fail_return_abstract.carbon:[[@LINE+7]]:3: error: initialization of abstract type `Abstract` [AbstractTypeInInit]
   // CHECK:STDERR:   return a;
   // CHECK:STDERR:   ^~~~~~~~~

toolchain/docs/check.md

@@ -415,7 +415,7 @@ process entries from the node stack until it finds that solo parse node from
 `ReturnStatementStart` that indicates it is done.
 
 Another pattern that arises is state is set up by an introducer node, updated by
-its siblings, and then consumed by the bracketing parent node. FIXME: example
+its siblings, and then consumed by the bracketing parent node. TODO: example
 
 ### Node stack
 

toolchain/docs/idioms.md

@@ -171,7 +171,7 @@ A fairly complete list of `ValueStore` uses should be available on
 
 ## Template metaprogramming
 
-FIXME: show example patterns
+TODO: show example patterns
 
 -   TypedInstArgsInfo from toolchain/sem_ir/inst.h
 -   templated using

toolchain/docs/parse.md

@@ -596,7 +596,7 @@ declaration or statement.
 
 ### Something required in context
 
-FIXME
+TODO
 
 Example: name after introducer
 [parse/handle_decl_name_and_params.cpp](/toolchain/parse/handle_decl_name_and_params.cpp)
@@ -796,7 +796,7 @@ deal with the `as` that follows when the type expression is present.
 
 ### Operators
 
-FIXME
+TODO
 
 An independent description of our approach:
 ["Better operator precedence" on scattered-thoughts.net](https://www.scattered-thoughts.net/writing/better-operator-precedence/)

toolchain/lower/file_context.cpp

@@ -100,9 +100,9 @@ auto FileContext::BuildDICompileUnit(llvm::StringRef module_name,
   llvm_module.addModuleFlag(llvm::Module::Max, "Dwarf Version", 5);
   llvm_module.addModuleFlag(llvm::Module::Warning, "Debug Info Version",
                             llvm::DEBUG_METADATA_VERSION);
-  // FIXME: Include directory path in the compile_unit_file.
+  // TODO: Include directory path in the compile_unit_file.
   llvm::DIFile* compile_unit_file = di_builder.createFile(module_name, "");
-  // FIXME: Introduce a new language code for Carbon. C works well for now since
+  // TODO: Introduce a new language code for Carbon. C works well for now since
   // it's something debuggers will already know/have support for at least.
   // Probably have to bump to C++ at some point for virtual functions,
   // templates, etc.
@@ -447,7 +447,7 @@ auto FileContext::BuildDISubprogram(const SemIR::Function& function,
   CARBON_CHECK(name, "Unexpected special name for function: {0}",
                function.name_id);
   auto loc = GetLocForDI(function.definition_id);
-  // FIXME: Add more details here, including real subroutine type (once type
+  // TODO: Add more details here, including real subroutine type (once type
   // information is built), etc.
   return di_builder_.createFunction(
       di_compile_unit_, *name, llvm_function->getName(),