[IRForTarget] Strenghten handling of alternate mangling.

Summary:
This fixes an issue with GCC generated binaries wherein an expression
with method invocations on std::string variables was failing. Such use
cases are tested in TestSTL (albeit, in a test marked with
@unittest2.expectedFailure because of other reasons).

The reason for this particular failure with GCC is that the generated
DWARF for std::basic_string<...> is incomplete, which makes clang not
to use the alternate mangling scheme. GCC correctly generates the name
of basic_string<...>:

DW_AT_name "basic_string<char, std::char_traits<char>, std::allocator<char> >"

It also lists the template parameters of basic_string correctly:

DW_TAG_template_type_parameter
    DW_AT_name                  "_CharT"
    DW_AT_type                  <0x0000009c>
DW_TAG_template_type_parameter
    DW_AT_name                  "_Traits"
    DW_AT_type                  <0x00000609>
DW_TAG_template_type_parameter
    DW_AT_name                  "_Alloc"
    DW_AT_type                  <0x000007fb>

However, it does not list the template parameters of std::char_traits<>.
This makes Clang feel (while parsing the expression) that the string
variable is not actually a basic_string instance, and consequently does
not use the alternate mangling scheme.

Test Plan:
dotest.py -C gcc -p TestSTL
          -- See it go past the "for" loop expression successfully.

Reviewers: clayborg, spyffe

Reviewed By: clayborg, spyffe

Subscribers: tberghammer, zturner, lldb-commits

Differential Revision: http://reviews.llvm.org/D8846

llvm-svn: 234522