[ELF] Export strong defined symbol if it coalesces away a weak symbol
defined in a shared library.

Now LLD does not export a strong defined symbol if it coalesces away a
weak symbol defined in a shared library. This bug affects all ELF
architectures and leads to segfault:

  % cat foo.c
  extern int __attribute__((weak)) flag;
  int foo() { return flag; }

  % cat main.c
  int flag = 1;
  int foo();
  int main() { return foo() == 1 ? 0 : -1; }

  % clang -c -fPIC foo.c main.c
  % lld -flavor gnu -target x86_64 -shared -o libfoo.so ... foo.o
  % lld -flavor gnu -target x86_64 -o a.out ... main.o libfoo.so
  % ./a.out
  Segmentation fault

The problem is caused by the fact that we lose all information about
coalesced symbols after the `Resolver::resolve()` method is finished.

The patch solves the problem by overriding the
`LinkingContext::notifySymbolTableCoalesce()` method and saving names
of coalesced symbols. Later in the `buildDynamicSymbolTable()` routine
we use this information to export these symbols.