Fix uninitialized value in AttributedTypeLoc.

Clang might crash while reading the precompiled headers if
we don't initialize the AttrEnumOperandLoc properly.

This commit fixes the combination of string attribute
operand and enum operand.  Besides, this commit also adds
several assertions to avoid unexpected operand kind.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203416 91177308-0d34-0410-b5e6-96231b3b80d8