Better scheme to lookup alternate mangled name when looking up function address.

Summary:
This change is relevant for inferiors compiled with GCC. GCC does not
emit complete debug info for std::basic_string<...>, and consequently, Clang
(the LLDB compiler) does not generate correct mangled names for certain
functions.

This change removes the hard-coded alternate names in
ItaniumABILanguageRuntime.cpp.

Before the hard-coded names were put in ItaniumABILanguageRuntime.cpp, one could
not evaluate std::string methods (ex. std::string::length). After putting in
the hard-coded names, one could evaluate them. However, it did not still
enable one to call methods on, say for example, std::vector<string>.
This change makes that possible.

There is some amount of incompleteness in this change. Consider the
following example:

std::string hello("hello"), world("world");
std::map<std::string, std::string> m;
m[hello] = world;

One can still not evaluate the expression "m[hello]" in LLDB. Will
address this issue in another pass.

Reviewers: jingham, vharron, evgeny777, spyffe, dawn

Subscribers: clayborg, dawn, lldb-commits

Differential Revision: http://reviews.llvm.org/D12809

llvm-svn: 257113