Fix two leaks found by LSan (one is test-only).

The result of llvm::MemoryBuffer::getMemBuffer() needs to be freed. Don't
pass "don't free" flag to overrideFileContents() to fix.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207075 91177308-0d34-0410-b5e6-96231b3b80d8