Correct definition of lvalues, fixes #10890

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
auto merge of #10982 : ezyang/rust/lvalue-def, r=pcwalton

Based off of Niko's proposed definition of lvalues.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Register new snapshots

Understand 'pkgid' in stage0. As a bonus, the snapshot now contains now metadata
(now that those changes have landed), and the snapshot download is half as large
as it used to be!
auto merge of #10919 : alexcrichton/rust/snapshots, r=brson

Understand 'pkgid' in stage0. As a bonus, the snapshot now contains now metadata
(now that those changes have landed), and the snapshot download is half as large
as it used to be!
Move logic for test output generation forward

By performing this logic very late in the build process, it ended up leading to
bugs like those found in #10973 where certain stages of the build process
expected a particular output format which didn't end up being the case. In order
to fix this, the build output generation is moved very early in the build
process to the absolute first thing in phase 2.

Closes #10973
AST Visitor now walks enum discriminant expression
Mark live codes in struct/enum for dead-code pass
auto merge of #10994 : ktt3ja/rust/issue-10956, r=alexcrichton

Types used inside live struct or enum are now marked live.

Fix #10956 and #10993.
debuginfo: Always set column information to zero for source locations.
debuginfo: Clear debug source locations at beginning of functions.
debuginfo: Clear source positions where they'd confuse LLVM otherwise.
debuginfo: Create separate lexical block for function bodies.
debuginfo: Get rid of redundant argument copying with debuginfo activated
debuginfo: Use same behavior as Clang for calculating scope_line parameter when creating function DIEs.
debuginfo: Set the is_local_to_unit attribute of functions correctly
debuginfo: Set correct source position for function calls.
debuginfo: Add test cases for function prologue handling.
debuginfo: Added some documentation of source location management.
debuginfo: Add comment on is_local_to_unit parameter.
vec: make the move iterator fast for all types

Closes #10976