Change the various AST transforms to use prebuilt DeclName constants more heavily rather than an ad-hoc mix of Identifiers, string literals, std::strings, and StringRefs with questionable memory management.
This change PCMacro and PlaygroundTransform to return an a moduleID and
fileID in addition to the source location information. The Frontend has
been changed to run PCMacro and PlaygroundTransform on all input files
instead of the main file only.
The tests have been updated to conform to these changes with an addition
of module and file ID specific tests. The Playgrounds related tests were
adjusted to make a module out of the stub interface files since those
files should not have PCMacro and PlaygroundTransform applied to them.
rdar://problem/50821146
Replace `NameOfType foo = dyn_cast<NameOfType>(bar)` with DRY version `auto foo = dyn_cast<NameOfType>(bar)`.
The DRY auto version is by far the dominant form already used in the repo, so this PR merely brings the exceptional cases (redundant repetition form) in line with the dominant form (auto form).
See the [C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es11-use-auto-to-avoid-redundant-repetition-of-type-names) for a general discussion on why to use `auto` to avoid redundant repetition of type names.
Based off the PlaygroundTransform, this new ASTWalker leaves calls to __builtin_pc_before and __builtin_pc_after before and after a user would expect a program counter to enter a range of source code.
