[mangle] Fix mangling where an extra mangle context is required.

Summary:
- [Itanium C++ ABI][1], for certain contexts like default parameter and
  etc., mangling numbering will be local to the particular argument in
  which it appears.
- However, for these cases, the mangle numbering context is allocated per
  expression evaluation stack entry. That causes, for example, two
  lambdas defined/used understand the same default parameter are
  numbered as the same value and, in turn, one of them is not generated
  at all.
- In this patch, an extra mangle numbering context map is maintained in
  the AST context to map taht extra declaration context to its numbering
  context. So that, 2 different lambdas defined/used in the same default
  parameter are numbered differently.

[1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html

Reviewers: rsmith, eli.friedman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68715

llvm-svn: 374200