[lldb][ClangExpression] Add asm() label to all FunctionDecls we create from DWARF

When resolving symbols during IR execution, lldb makes a last effort attempt
to resolve external symbols from object files by approximate name matching.
It currently uses `CPlusPlusNameParser` to parse the demangled function name
and arguments for the unresolved symbol and its candidates. However, this
hand-rolled C++ parser doesn’t support ABI tags which, depending on the demangler,
get demangled into `[abi:tag]`. This lack of parsing support causes lldb to never
consider a candidate mangled function name that has ABI tags.

The issue reproduces by calling an ABI-tagged template function from the
expression evaluator. This is particularly problematic with the recent
addition of ABI tags to numerous libcxx APIs.

The issue stems from the fact that `clang::CodeGen` emits function
function calls using the mangled name inferred from the `FunctionDecl`
LLDB constructs from DWARF. Debug info often lacks information for
us to construct a perfect FunctionDecl resulting in subtle mangled
name inaccuracies.

This patch side-steps the problem of inaccurate `FunctionDecl`s by
attaching an `asm()` label to each `FunctionDecl` LLDB creates from DWARF.
`clang::CodeGen` consults this label to get the mangled name as one of
the first courses of action when emitting a function call.

LLDB already does this for C++ member functions as of
[675767a5](https://reviews.llvm.org/D40283)

**Testing**

* Added API tests

Differential Revision: https://reviews.llvm.org/D131974