Fix enum LanguageType values and language string table lookups.

Summary:
* Fix enum LanguageType values so that they can be used as indexes
into array language_names and g_languages as assumed by
LanguageRuntime::GetNameForLanguageType,
Language::SetLanguageFromCString and Language::AsCString.
* Add DWARFCompileUnit::LanguageTypeFromDWARF to convert from DWARF
DW_LANG_* values to enum LanguageType values.

Reviewed By: clayborg, abidh
Differential Revision: http://reviews.llvm.org/D10484

llvm-svn: 239963