[AST] Initialized data after TypeSourceInfo

There is no initialization of the data between allocation
and first getBeginLoc call.

allocation: llvm-project/clang/lib/AST/ASTContext.cpp:3022
use: llvm-project/clang/lib/AST/TypeLoc.cpp:222

Msan report https://reviews.llvm.org/P8306

Reviewed By: thurston

Differential Revision: https://reviews.llvm.org/D150499