[clang-format] Add IndentPragma style to eliminate common clang-format off scenario

A quick search of github.com, shows one common scenario for excessive use of //clang-format off/on is the indentation of #pragma's, especially around the areas of loop optimization or OpenMP

This revision aims to help that by introducing an `IndentPragmas` style, the aim of which is to keep the pragma at the current level of scope

```
    for (int i = 0; i < 5; i++) {
// clang-format off
        #pragma HLS UNROLL
        // clang-format on
        for (int j = 0; j < 5; j++) {
// clang-format off
            #pragma HLS UNROLL
            // clang-format on
     ....
```

can become

```
    for (int i = 0; i < 5; i++) {
        #pragma HLS UNROLL
        for (int j = 0; j < 5; j++) {
            #pragma HLS UNROLL
        ....
```

This revision also support working alongside the `IndentPPDirective` of `BeforeHash` and `AfterHash` (see unit tests for examples)

Reviewed By: curdeius

Differential Revision: https://reviews.llvm.org/D92753