Fix use-after-destruction introduced in r224924.

getMainExecutable() returns a std::string, assigning its result
to StringRef immediately creates a dangling pointer. This was
detected by half-broken fast-MSan-bootstrap bot.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224956 91177308-0d34-0410-b5e6-96231b3b80d8