which has better portability across different OSes.
In addition, on many ELF OSes, this output relocatable file can be
linked without -no-pie.

This will make it easy for callers to see issues with and fix up calls
to createTargetMachine after a future change to the params of
TargetMachine.

This matches other nearby enums.

For downstream users, this should be a fairly straightforward
replacement,
e.g. s/CodeGenOpt::Aggressive/CodeGenOptLevel::Aggressive
or s/CGFT_/CodeGenFileType::

There were quite a few places in the tutorial where the snippets were not up to date with the full code listings given. I have updated all of the ones I could find, which mostly involves changing `.` to `->` when accessing through a `std::unique_ptr`, changing `while (1)` to `while (true)`, and other such small changes.

There are still however a few places where I am not sure what to do, such as:
- Chapter 4: `ParseTopLevelExpr()` in chapter 3 sets the ProtoTypeAST
             name to "", however it is referred to as "__anon_expr"
             in chapter 4. Would it be required to mention this change
             in chapter 4?
- Chapter 9: The code snippets refer to the top level expression as
             "main", however the full code listing refers to it as
             "__anon_expr". I think given the context of the chapter
             it makes sense to refer to it as "main", so I have
             updated the code listing to reflect that.
- Chapter 9: In chapter 9 the diff given for `HandleTopLevelExpression()`
             deletes code that is not mentioned anywhere else, so I am
             not sure what is to be done there.
- Miscellaneous: I don't think this is very important, however the
                 casing used for the first word of error messages
                 tends to vary between upper and lower case between
                 chapters and I do not know if it is worth reconciling
                 these differences.

Reviewed By: xgupta

Differential Revision: https://reviews.llvm.org/D142323

Unorphan the old tutorial and reference every page in the index
explicitly. This should hopefully make Sphinx generate correct
hyperlinks now.

Avoids the need to include TargetMachine.h from various places just for
an enum. Various other enums live here, such as the optimization level,
TLS model, etc. Data suggests that this change probably doesn't matter,
but it seems nice to have anyway.

F_{None,Text,Append} are kept for compatibility since r334221.

llvm-svn: 367800

llvm-svn: 366528