Initialize the "IsCompleteDefinitionRequired" field in the ASTContext ctor

Introduced in r186262 & found by the hexagon buildbots (but owing to
this being UB, that's random chance - so there's no additional test case
here)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186265 91177308-0d34-0410-b5e6-96231b3b80d8