Some minor improvements on the symtab parsing code

* Remove an unneccessary re-computaion on arch spec from the ELF file
* Use a local cache to optimize name based section lookups in symtab
  parsing
* Optimize C++ method basename validation with replacing a regex with
  hand written code

These modifications reduce the time required to parse the symtab from
large applications by ~25% (tested with LLDB as inferior)

Differential revision: http://reviews.llvm.org/D14088

llvm-svn: 251402