Backend support for top-level Clang epxressions

This patch adds a new ExecutionPolicy, eExecutionPolicyTopLevel, which
tells the expression parser that the expression should be JITted as top
level code but nothing (except static initializers) should be run.  I
have modified the Clang expression parser to recognize this execution
policy.  On top of the existing patches that support storing IR and
maintaining a map of arbitrary Decls, this is mainly just patching up a
few places in the expression parser.

I intend to submit a patch for review that exposes this functionality
through the "expression" command and through the SB API.  That patch
also includes a testcase for all of this.

<rdar://problem/22864976>

llvm-svn: 264095