#include file pathnames are being processed for escapes

### Problematic code:
**HLSL** shader with this:
```hlsl
#include "shared\common.h"
```
### Expected behaviour
Successful preprocessing and compilation.
### Current behaviour:
Preprocessing errors:
```
ERROR: common.h:4: 'string' : Invalid escape sequence
ERROR: common.h:4: 'string' : End of line in string
```

### glslang version
Built from commit 3bf1dab23d9a3af0c93ce4e717f13417df12606a

#### Misc
Seems to be related with #2108.

Start by reproducing the HLSL example with glslang at the referenced commit and trace how the preprocessor handles the include pathname. The work is done when `shared\common.h` preprocesses and compiles successfully without invalid-escape or end-of-line string errors.

Written by the indexing model from the issue text.