Only capture the SIMD loop body.

- Extended the LoopInfo and LoopInfoStack to mark
  the captured loop body as parallel.
Allow multiple _Cilk_spawn calls in a single DeclStmt

We now serialize the _Cilk_spawn calls in case of a DeclStmt that
contains multiple VarDecls.
Outline for SIMDForScopeInfo to hold Inner & Outer variables + constructed Update Expressions.
Create local variables for private variables.
Move cilk_for and simd Sema code into SemaCilk.cpp
SemaCilk cleanup

- merge IsValidCilkForIncrement and CheckSIMDForIncrement into
  CheckForIncrement since they are almost the same.
SemaCilk refactoring

- merge ExtractCilkForCondition and ExtractSIMDForCondition into
  ExtractForCondition.
SemaCilk refactoring

- merge CheckCilkForCondition and CheckSIMDForCond into
  CheckForCondition.
SemaCilk refactoring

- merge ChecCilkForInitStmt and CheckSIMDForInit into
  CheckForInit
Construct init expression for all simd clauses, and update expression for lastprivate.
AST and Codegen for private variables

- A sanity test is included and more tests will follow.
Remove forgotten debug output.
SemaCilk refactoring

- merge ActOnSIMDForStmt and ActOnCilkForStmt
- merge ActOnSIMDForStmtError and ActonCilkForStmtError
Handle continue statements inside a SIMD for
Add LoopCount expression to SIMDForStmt
Fix a few small issues

- variables in both private and {first, last}private are not allowed
- codegen firstprivate and private variable only and disable others
Emit SIMD for loop index

- not used yet
Added more codegen tests for private variables

- sanity check for firstprivate codegen, since firstprivate
  will be deprecated.
Don't generate invoke instrs inside a simd loop
- Allow exceptions to be disabled in CodeGenFunction
- Disable exceptions when emitting the simd loop helper function and
  when emitting the call to the simd loop function
Emit an update expression for each lastprivate variable.