[AMD Public Use]

Hello Everyone,

Requesting review on this patch, Please have a look.

Problem Description/Summary:
- clang emits DWARFv5 conformant line tables(ie. .debug_line section) which makes 2 things explicitly:
1. The current directory is explicitly present in the directories field and has index 0.
2. The current compilation filename is explicitly present and has index 0.

This leads to a subtle issue in GDB while accessing macro information:
Consider the following program:
```
#define OUT 4
int main() { int a = OUT;}
```
Compiled as `clang -gdwarf-5 -fdebug-macro test.c`
GDB session:
```
$gdb a.out -q
....
2       int main() { int a = OUT;}
(gdb) info macro OUT
The symbol `OUT' has no definition as a C/C++ preprocessor macro
at /home/test.c:-1
```

This issue cannot be reproduced using GCC compiler as GCC doesnot emit DWARFv5 compliant tables even
At `-gdwarf-5` flag.

Issue is stemming from line-header.c. When performing a file name lookup for macro filename GDB 
mistakenly appends FULL path after checking following condition, which as **one may notice** for 
directory index `0` will always evaluates `true` for DWARv5 line tables and `false` for DWARFv4 line tables.
```
If (dir != NULL)
return gdb::unique_xmalloc_ptr<char> (concat (dir, SLASH_STRING,
                                                          fe->name,
                                                          (char *) NULL));
```
Result of this, macro `filename` getting populated with fullname. Eventually macro lookup
results in failure due to mismatch in `main_file->filename` and   `sal.symtab->filename`.

This patch attempts to fix above problem by extending the check for file `0`, i.e for file `0`
do not append the fullname.

Testing:
- NO regressions in GDB testsuite before and after the fix.

We also tried add a test case for this fix, but was unsuccessful.
GDB test suite uses full path to the test files and this specific problem is 
not reproducible when user specifies full path. i.e
Not reproducible cases:
1. `clang -gdwarf-5 -fdebug-macro ./test.c` // macro visible inside GDB
2. `clang -gdwarf-5 -fdebug-macro /ABSOLUTE_PATH/test.c` // macro visible inside GDB

Reproducible:
1. `clang -gdwarf-5 -fdebug-macro test.c` //log shown above.

gdb/ChangeLog
2021-03-26  Sourabh Singh Tomar  <SourabhSingh.Tomar@amd.com>

       * dwarf2/line-header.c: Updated the comment and extended
       check in function `line_header::file_file_name` for
       DWARFv5 line tables.

Thank You,
Sourabh.