Fix breakpoint language filtering for other C variants (like C99) and Pascal.

This patch fixes setting breakpoints on symbol for variants of C and
Pascal where the language is "unknown" within the filter-by-language
process added in r252356. It also renames GetLanguageForSymbolByName to
GuessLanguageForSymbolByName and adds comments explaining the pitfalls
of the flawed assumption that the language can be determined solely from
the name and target.

Reviewed by: jingham
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D15175

llvm-svn: 254753