[ASTMatchers] Add support of hasCondition for SwitchStmt.

Summary:
The switch statement could be added to the hasCondition matcher.

Example:
```
clang-query> match switchStmt(hasCondition(ignoringImpCasts(declRefExpr())))
```

Output:
```
Match #1:

Binding for "root":
SwitchStmt 0x2f9b528 </usr/local/google/home/etienneb/examples/enum.cc:35:3, line:38:3>
|-<<<NULL>>>
|-ImplicitCastExpr 0x2f9b510 <line:35:11> 'int' <IntegralCast>
| `-ImplicitCastExpr 0x2f9b4f8 <col:11> 'enum Color' <LValueToRValue>
|   `-DeclRefExpr 0x2f9b4d0 <col:11> 'enum Color' lvalue Var 0x2f9a118 'C' 'enum Color'
`-CompoundStmt 0x2f9b610 <col:14, line:38:3>
  |-CaseStmt 0x2f9b578 <line:36:3, col:22>
  | |-ImplicitCastExpr 0x2f9b638 <col:8> 'int' <IntegralCast>
  | | `-DeclRefExpr 0x2f9b550 <col:8> 'enum Size' EnumConstant 0x2f99e40 'Small' 'enum Size'
  | |-<<<NULL>>>
  | `-ReturnStmt 0x2f9b5d0 <col:15, col:22>
  |   `-IntegerLiteral 0x2f9b5b0 <col:22> 'int' 1
  `-DefaultStmt 0x2f9b5f0 <line:37:3, col:12>
    `-BreakStmt 0x2f9b5e8 <col:12>

1 match.
```

Reviewers: aaron.ballman, sbenza, klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D20767

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271208 91177308-0d34-0410-b5e6-96231b3b80d8