[flang][docs] Update -fbackslash option for PGI compilers

[FruitClover]

Summary

PGI has reversed default option for escapes in literals compared to
flang/gfortan/Intel, note that and also that GNU has extension for "\"
interptetation:

$ cat f.f90
subroutine f()
  character, parameter :: x = "\"
end

gfortran:

$ gfortran f.f90 -fsyntax-only -std=f2018
$ gfortran f.f90 -fsyntax-only -std=f2018 -fbackslash
f.f90:2:33:

    2 |   character, parameter :: x = "\"
      |                                 1
Warning: Extension: backslash character at (1)

nvfortan:

$ nvfortran f.f90 -Msyntax-only
NVFORTRAN-S-0026-Unmatched quote (f.f90: 2)
  0 inform,   0 warnings,   1 severes, 0 fatal for f
$ nvfortran f.f90 -Msyntax-only -Mbackslash

flang:

$ flang-new f.f90 -fsyntax-only
$ flang-new f.f90 -fsyntax-only -fbackslash
error: Could not scan f.f90
./f.f90:2:31: error: Incomplete character literal
    character, parameter :: x = "\"
                                ^^^

[FruitClover]

Upd

[FruitClover]

Fix typo

[FruitClover]

gentle ping

[PeteSteinfeld]

Sorry for the delay. I didn't notice this the first time around.

Thanks for doing this!

flang/docs/OptionComparison.md

<table>
    <td>N/A
    </td>
    <td>fbackslash
    </td>
    <td><a href="https://www-01.ibm.com/support/docview.wss?uid=swg27024803&aid=1#page=163">qescape</a>
    </td>
    <td><a href="https://software.intel.com/en-us/fortran-compiler-developer-guide-and-reference-assume">assume bscc</a>
    </td>
-   <td>Mbackslash
+   <td><a href="https://docs.nvidia.com/hpc-sdk/compilers/hpc-compilers-ref-guide/index.html#cmdln-m-opts-fort-lang">Mnobackslash</a>
    </td>
-   <td>Mbackslash
+   <td>fbackslash
    </td>
   </tr>
   <tr>
    <td>Allow multibyte characters in strings
    </td>
    <td>N/A
    </td>
    <td>N/A
*   **PGI, Flang:**
     * in fixed form with -Mextend, characters after column 132 are ignored
 
 **<a name="names"></a>Names, Literals, and other tokens**
 
 **Escapes in literals:**
 
 
 *   **GNU:** The "-fbackslash" option the interpretation of backslashes in string literals from a single backslash character to "C-style" escape characters. The following combinations are expanded \a, \b, \f, \n, \r, \t, \v, \\, and \0 to the ASCII characters alert, backspace, form feed, newline, carriage return, horizontal tab, vertical tab, backslash, and NUL, respectively. Additionally, \xnn, \unnnn and \Unnnnnnnn (where each n is a hexadecimal digit) are translated into the Unicode characters corresponding to the specified code points. All other combinations of a character preceded by \ are unexpanded.
+
+    **Note** that with -fbackslash \ interpretetation as \ (single backslash character) is a **GNU** extension (-std=gnu).
+
 *   **Intel:** The option "-assume bscc" tells the compiler to treat the backslash character (\) as a C-style control (escape) character syntax in character literals. "nobscc" specifies that the backslash character is treated as a normal character in character literals.  This is the default.
 
+*   **PGI:** -Mbackslash instructs the compiler to treat the backslash as a normal character, and not as an escape character in quoted strings. Default is -Mnobackslash.
+
 **"$" in symbol names:** Allowing "$" in names is controlled by an option in GNU and is the default behavior in IBM and Intel.  Presumably, these compilers issue warnings when standard conformance options are enabled.  Dollar signs in names don't seem to be allowed in Cray, PGI, or Flang.
 
+**Note** that **PGI** has different default option for escapes in literals (-Mnobackslash) compared to **GNU**/**Intel**.
+
 **<a name="do"></a>DO loop handling**
 
 **One trip:**
 
 
 
 *   **IBM:** IBM has two options that do the same thing: "-1" and "-qonetrip".
 *   **Intel:** Intel used to support a "-onetrip" option, but it has been removed.  Intel now supports a "-f66" option that ensures that DO loops are executed at least once in addition to [several other Fortran 66 semantic features](https://software.intel.com/en-us/fortran-compiler-developer-guide-and-reference-f66#320D769C-7C41-4A84-AE0E-50A72296A838).