[clang-format] Mark pragma region lines as StringLiterals

In our code-base we auto-generate pragma regions the regions
look like method signatures like:

`#pragma region MYREGION(Foo: bar)`

The problem here was that the rest of the line after region
was not marked as stringliteral as in the case of pragma mark
so clang-format tried to change the formatting based on the
method signature.

Added test and mark it similar as pragma mark.

Reviewed By: owenpan

Differential Revision: https://reviews.llvm.org/D136336