[clang-tidy] ClangTidy check to flag uninitialized builtin and pointer fields.

[clang-tidy] ClangTidy check to flag uninitialized builtin and pointer fields.

Summary:
This patch is a continuation of http://reviews.llvm.org/D10553 by Jonathan B Coe.

The main additions are:

1. For C++11 the check suggests in-class field initialization as fix. This
makes the fields future proof towards the addition of new constructors.
2 For older language versions the fields are added in the right position
in the initializer list with more tests.
3. User documentation.

Reviewers: alexfh, jbcoe

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16517

llvm-svn: 260873