Fixed a problem where we couldn't call extern "C" functions.

Some compilers do not mark up C++ functions as extern "C" in the DWARF, so LLDB
has to fall back (if it is about to give up finding a symbol) to using the base
name of the function.

This fix also ensures that we search by full name rather than "auto," which
could cause unrelated C++ names to be found.  Finally, it adds a test case.

<rdar://problem/25094302>

llvm-svn: 271551