[ELF] - Fix incorrect logic in VersionScriptParser::parseVersion()

Previously the next sample script would generate 2 entries in 
Config->SymbolVersions with the same version name.

VERSION {
 global: c;
};
That happened because parseVersionSymbols() was called twice.
At first for "global:" and since there is no local tag, it was called again.
Patch fixes the issue, testcase was updated to demonstrate.

Differential revision: http://reviews.llvm.org/D21640

llvm-svn: 273663