Teach the allocation function overload handling to deal with templates, and

prevent a crash on templates when looking for an existing declaration of the
predefined global operators. This fixes PR5918.

Added an easy test case for the overload handling, but testing the crash is
a bit trickier. Created a new test that can use multiple runs with a define to
trigger which test case is used so we can test this type of issue.

llvm-svn: 95220