Initialize StandardConversionSequence correctly

MSAN detected a path that leaves DeprecatedStringLiteralToCharPtr uninitialized.
UserDefinedConversionSequence::First is a StandardConversionSequence that must
be initialized with setAsIdentityConversion.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199988 91177308-0d34-0410-b5e6-96231b3b80d8