Refactored int/uint range code in preparation for change to range_rev semantics.

Also added unit tests of range code to test refactoring.  The
num-range-rev.rs test will need to be updated when the range_rev
semantics change.
Switch over to new range_rev semantics; fix #5270.
Fix #5270: another test I did not update properly.
syntax: make a macros-injection pass; conditionally define debug! to a noop based on cfg(debug).

Macros can be conditionally defined because stripping occurs before macro
expansion, but, the built-in macros were only added as part of the actual
expansion process and so couldn't be stripped to have definitions conditional
on cfg flags.

debug! is defined conditionally in terms of the debug config, expanding to
nothing unless the --cfg debug flag is passed (to be precise it expands to
`if false { normal_debug!(...) }` so that they are still type checked, and
to avoid unused variable lints).
Add doc and examples for attributes allow/warn/deny/forbid.

Fix #7821.
Clean-up tests after debug!/std-macros change.

The entire testsuite is converted to using info! rather than debug!
because some depend on the code within the debug! being trans'd.
rustc: handle allocas and LoadRangeAsserts in unreachable blocks correctly.

An alloca in an unreachable block would shortcircuit with Undef, but with type
`Type`, rather than type `*Type` (i.e. a plain value, not a pointer) but it is
expected to return a pointer into the stack, leading to confusion and LLVM
asserts later.

Similarly, attaching the range metadata to a Load in an unreachable block
makes LLVM unhappy, since the Load returns Undef.

Fixes #7344.
rustc: selectively trans branches for `if <literal-bool>`.

That is, the `b` branch in `if true { a } else { b }` will not be
trans'd, and that expression will be exactly the same as `a`. This
means that, for example, macros conditionally expanding to `if false
{ .. }` (like debug!) will not waste time in LLVM (or trans).
auto merge of #7822 : huonw/rust/cond-debug, r=graydon

As per @pcwalton's request, `debug!(..)` is only activated when the `debug` cfg is set; that is, for `RUST_LOG=some_module=4 ./some_program` to work, it needs to be compiled with `rustc --cfg debug some_program.rs`. (Although, there is the sneaky `__debug!(..)` macro that is always active, if you *really* need it.)

It functions by making `debug!` expand to `if false { __debug!(..) }` (expanding to an `if` like this is required to make sure `debug!` statements are typechecked and to avoid unused variable warnings), and adjusting trans to skip the pointless branches in `if true ...` and `if false ...`.

The conditional expansion change also required moving the inject-std-macros step into a new pass, and makes it actually insert them at the top of the crate; this means that the cfg stripping traverses over the macros and so filters out the unused ones.

This appears to takes an unoptimised build of `librustc` from 65s to 59s; and the full bootstrap from 18m41s to 18m26s on my computer (with general background use).

`./configure --enable-debug` will enable `debug!` statements in the bootstrap build.
auto merge of #7823 : pnkfelix/rust/issue7821-document-lint-attributes, r=graydon

r? anyone

Fix #7821.
auto merge of #7684 : pnkfelix/rust/fsk-invert-range-rev-halfclosedness-issue5270-2ndpr, r=cmr

Changes int/uint range_rev to iterate over range `(hi,lo]` instead of `[hi,lo)`.

Fix #5270.

Also:
* Adds unit tests for int/uint range functions
* Updates the uses of `range_rev` to account for the new semantics.  (Note that pretty much all of the updates there were strict improvements to the code in question; yay!)
* Exposes new function, `range_step_inclusive`, which does the range `[hi,lo]`, (at least when `hi-lo` is a multiple of the `step` parameter).
* Special-cases when `|step| == 1` removing unnecessary bounds-check.  (I did not check whether LLVM was already performing this optimization; I figure it would be a net win to not leave that analysis to the compiler.  If reviewer objects, I can easily remove that from the refactored code.)

(This pull request is a rebased version of PR #7524, which went stale due to recent unrelated changes to num libraries.)