[OMPT] Fix return address handling in a few GOMP interface methods

This revision fixes failing testcases with parallel for loops and the gomp
interface. The return address needs to be stored at entry to runtime.
The storage is cleared on usage, so we need to update the storage before
calling again internal functions, that will trigger event callbacks.

Differential Revision: https://reviews.llvm.org/D41181

llvm-svn: 321265