[analyzer] Highlight c_str() call in DanglingInternalBufferChecker.

Add a bug visitor to DanglingInternalBufferChecker that places a note
at the point where the dangling pointer was obtained. The visitor is
handed over to MallocChecker and attached to the report there.

Differential Revision: https://reviews.llvm.org/D48522

llvm-svn: 336495