Fix completion for functions in anonymous namespaces

I was going through some of the old bugs and came across PR21069 which I
was able to reproduce. The issue is that we match the regex `^foo`
against the `DW_AT_name` in the DWARF, which for our anonymous function
is indeed `foo`. However, when we get the function name from the symbol
context, the result is `(anonymous namespace)::foo()`. This throws off
completions, which assumes that it's appending to whatever is already
present on the input, resulting in a bogus
`b fooonymous\ namespace)::foo()`.

Bug report: https://llvm.org/PR21069

Differential revision: https://reviews.llvm.org/D65498

llvm-svn: 367455