Clarify when we can avoid creating names for temp symbols.

Some temporary symbols are created by MC itself. These symbols are never used
for lookup and are never included in the object symbol table, so we can
avoid creating a name for them.

Other temporaries are created by CodeGen or by the user by explicitly asking
for a name starting with .L (or L on MachO).

These temporaries behave like regular symbols, we just try to avoid including
them in the object symbol table, but sometimes they end up there:

const char *foo() {
  return "abc" + 3;
}

will have a relocation pointing to a .L symbol.

It just so happens that almost all MC created temporary has the AlwaysAddSuffix
option and CodeGen/user created ones don't.

One interesting future optimization would be to use unnamed symbols for
all temporaries, but that would require use an st_name of 0 or
having the object writer create the names if a symbol does end up in the
symbol table.

No testcase since this just avoid creating a few extra names for MC created
temporaries.

llvm-svn: 238887