[PATCH] Add -Wrange-loop-analysis to warn when a range-based for-loop is creating a copy.

[Richard Trieu]

This warning will trigger on loop variables that make copies in a range-based for-loop.  The three cases this warning catches are:

1) for (const Foo &x : Foos), where the range Foos does not return a copy.  This warning will suggest using the non-reference type so the copy is obvious.
2) for (const Foo x : Foos), where the range Foos does return a reference, but is copied into x.  This warning will suggest using the reference type to prevent a copy from being made.
3) for (const Bar &x : Foos), where Bar is constructed from Foo.  In this case, suggest using the non-reference "const Bar" to indicate a copy is intended to be made, or "const Foo &" to prevent a copy from being made.

-Wrange-loop-analysis is being added as a sub-group to -Wloop-analysis.  The previous warnings there are moved to -Wfor-loop-analysis.  While the warnings are currently split along statement types, a finer grain division of warnings may be needed.

http://reviews.llvm.org/D4169

Files:
  include/clang/Basic/DiagnosticGroups.td
  include/clang/Basic/DiagnosticSemaKinds.td
  lib/Sema/SemaStmt.cpp
  test/SemaCXX/warn-range-loop-analysis.cpp
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