Adds conditional byteswapping intrinsics

These intrinsics are synthesized, so maybe they should be in another
file. But since they are just a single line of code each, based on the
bswap intrinsics and aren't really intended for public consumption (they should be exposed as a
single function / trait) I thought they would fit here.
rt: fix jemalloc android cross-compile for mac
Cleanup src/librustc/back
syntax::diagnostics: Color the ^~~~ in green for better visibility

Fixes #7164.
Consume once fns when calling them (#2549).
Permit moving out of captured upvars in once fns. Close #2549.
Add tests for once functions (#2549).
Hide the once-move-out privilege for stack fns behind '-Z once-fns'
Oops, hide 'unless once fn' error message hint behind -Z once-fns too.
Added overview of the debuginfo module at the beginning of debuginfo.rs.
Fixed compiler warnings about unnecessary string allocations in debuginfo.rs
Reordered debuginfo.rs and commented public fns
debuginfo.rs: Removed some static constants not used anymore.
debuginfo.rs: Explained why a function is called although one could think it should not be.
debuginfo.rs: Removed trailing whitespace.
extra: Add a testcase for #7256.
Avoid unnecessary scratch datums for by-copy function arguments

Currently, by-copy function arguments are always stored into a scratch
datum, which serves two purposes.  First, it is required to be able to
have a temporary cleanup, in case that the call fails before the callee
actually takes ownership of the value. Second, if the argument is to be
passed by reference, the copy is required, so that the function doesn't
get a reference to the original value.

But in case that the datum does not need a drop glue call and it is
passed by value, there's no need to perform the extra copy.
Make task_rng() return @mut R instead of @@mut R
librustc: Don't allow enum struct variants to shadow structs.
Add test for duplicate definitions of structs and enum struct variants.