Fix 2 cases of uninitialized reads of an invalid PresumedLoc.

The code in CGExpr was added back in 2012 (r165536) but not exercised in tests
until recently.

Detected on the MemorySanitizer bootstrap bot.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190521 91177308-0d34-0410-b5e6-96231b3b80d8