[SemaExprCXX] Avoid calling isInSystemHeader for invalid source locations

Summary:
While diagnosing a CXXNewExpr warning, we were calling isInSystemHeader(), which expect to be
called with a valid source location. This causes an assertion failure if the location is unknown.
A quick grep shows it's not without precedent to guard calls to the function with a
"Loc.isValid()".

This fixes a test failure in LLDB, which always creates object with invalid source locations as it
does not (always) have access to the source.

Reviewers: nlewycky

Subscribers: lldb-commits, cfe-commits

Differential Revision: http://reviews.llvm.org/D17847

llvm-svn: 262700