Give builtins and alloc/dealloc operators the default calling convention.

Give builtins and alloc/dealloc operators the default calling convention.

On SPIR targets, the default calling convention is SpirFunction.
However, operator new/delete and builtins were being created with CC_C.
The result is indirect references to new/delete (or builtins that are permitted
to be called indirectly have a mismatched type, as well as questionable codegen
in some cases.

This patch sets both to the default calling convention, so that it
properly matches the calling convention of the target.

Differential Revision: https://reviews.llvm.org/D58844

Change-Id: I52065bb00bc2655945caea8f29c409ba1e0ac24a
llvm-svn: 355317