[OpenMP] Rename the offload entry points.

Summary:
Unlike other outlined regions in OpenMP, offloading entry points have to have be visible (external linkage) for the device side. Using dots in the names of the entries can be therefore problematic for some toolchains, e.g. NVPTX.

Also the patch drops the column information in the unique name of the entry points. The parsing of directives ignore unknown tokens, preventing several target  regions to be implemented in the same line. Therefore, the line information is sufficient for the name to be unique. Also, the preprocessor printer does not preserve the column information, causing offloading-entry detection issues if the host uses an integrated preprocessor and the target doesn't (or vice versa).

Reviewers: hfinkel, arpith-jacob, carlo.bertolli, kkwli0, ABataev

Subscribers: cfe-commits, fraggamuffin, caomhin

Differential Revision: http://reviews.llvm.org/D17179

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260837 91177308-0d34-0410-b5e6-96231b3b80d8