Add support for parsing and representing C++ constructor declarations.

Add support for parsing and representing C++ constructor declarations.

Notes:
  - Constructors are never found by name lookup, so they'll never get
    pushed into any scope. Instead, they are stored as an
    OverloadedFunctionDecl in CXXRecordDecl for easy overloading.
  - There's a new action isCurrentClassName that determines whether an
    identifier is the name of the innermost class currently being defined;
    we use this to identify the declarator-id grammar rule that refers to
    a type-name.
  - MinimalAction does *not* support parsing constructors.
  - We now handle virtual and explicit function specifiers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58499 91177308-0d34-0410-b5e6-96231b3b80d8