Fix PR30440: Initialize FunctionTypeDepth in CXXNameMangler

This commit fixes PR 30440 by initializing CXXNameMangler's FunctionTypeDepth
in the two constructors added in r274222 (The commit that caused this
regression).

rdar://28455269

Differential Revision: https://reviews.llvm.org/D24932


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283428 91177308-0d34-0410-b5e6-96231b3b80d8