[DebugInfo][CodeView] Use <lambda_n> as the display name for lambdas.

Currently (for codeview) lambdas have a string like `<lambda_0>` in
their mangled name, and don't have any display name. This change uses the
`<lambda_0>` as the display name, which helps distinguish between lambdas
in -gline-tables-only, since there are no linkage names there.
It also changes how we display lambda names; previously we used
`<unnamed-tag>`; now it will show `<lambda_0>`.

I added a function to the mangling context code to create this string;
for Itanium it just returns an empty string.

Bug: https://bugs.llvm.org/show_bug.cgi?id=48432

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D95187