[OCaml] Fix incorrect use of CAMLlocal in nested blocks

Summary:
The OCaml manual states:

> Local variables of type value must be declared with one of the
> CAMLlocal macros. [...] These macros must be used at the beginning
> of the function, not in a nested block.

This patch moves several instances of CAMLlocal macros from nested
blocks to the function beginning.

Reviewers: whitequark

Reviewed By: whitequark

Subscribers: CodaFi, llvm-commits

Differential Revision: https://reviews.llvm.org/D53841

llvm-svn: 346387