[ObjC] Avoid a @try/@finally/@autoreleasepool fixit when parsing an expression

This patch ensures that the typo fixit for the @try/@finally/@autoreleasepool {}
directive is shown only when we're parsing an actual statement where such
directives can actually be present.

rdar://19669565

Differential Revision: https://reviews.llvm.org/D26916

llvm-svn: 288334