Apply re-formatting to C++ in src/ tree.

Apply re-formatting to C++ in src/ tree.

Apply formatting changes in bulk to reduce noise in subsequent
functional changes.

Will likely need to be repeated if clang-format version is updated
per #3708.

The formatting is sensitive to differences even between llvm tools
versions 7.1.0.2 and 8.0.1.3. Here, I used 7.1.

Refs #3833