[LLD][PATCH] Do not keep STT_FILE symbols in the symbol table

[Simon Atanasyan via llvm-commits]

Hi,

Now LLD keeps STT_FILE symbols in the symbol table because
`elf2::shouldKeepInSymtab` returns true for them. But if such symbol
has an empty name, the LLD crashes.

% cat test.s
 .file ""
 .globl _start;
 _start:
   mov $60, %rax
   mov $42, %rdi
   syscall
% llvm-mc -filetype=obj -triple=x86_64-unknown-linux test.s -o test.o
% lld -flavor gnu test.o

The problem is in the `elf2::shouldKeepInSymtab`. STT_FILE symbol has
no associated section so the `File.getSection(Sym)` returns null and
the LLD crashes on the `Sec->getSectionHdr()->sh_flags` call.

The most simple fix is to ignore STT_FILE symbols at all. Attached
patch implements that.

But maybe we want to keep them for some reasons?

-- 
Simon Atanasyan
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