Add warning capabilities in LLVM.
This reapplies r197438 and fixes the link-time circular dependency between
IR and Support. The fix consists in moving the diagnostic support into IR.

The patch adds a new LLVMContext::diagnose that can be used to communicate to
the front-end, if any, that something of interest happened.
The diagnostics are supported by a new abstraction, the DiagnosticInfo class.
The base class contains the following information:
- The kind of the report: What this is about.
- The severity of the report: How bad this is.

This patch also adds 2 classes:
- DiagnosticInfoInlineAsm: For inline asm reporting. Basically, this diagnostic
will be used to switch to the new diagnostic API for LLVMContext::emitError.
- DiagnosticStackSize: For stack size reporting. Comes as a replacement of the
hard coded warning in PEI.

This patch also features dynamic diagnostic identifiers. In other words plugins
can use this infrastructure for their own diagnostics (for more details, see
getNextAvailablePluginDiagnosticKind).

This patch introduces a new DiagnosticHandlerTy and a new DiagnosticContext in
the LLVMContext that should be set by the front-end to be able to map these
diagnostics in its own system.

http://llvm-reviews.chandlerc.com/D2376
<rdar://problem/15515174>