Revert one part of Ashok Thirumurthi's patch from September 16, r190812.

In those set of patches, Ashok changed Module::ResolveSymbolContextForAddress
so that if it failed to find a symbol for a pc, it could back up
the pc value by 1 and re-search for a symbol.

His change to RegisterContextLLDB.cpp partially duplicates that
behavior but it also removes the separate case where we find a
Symbol for the pc address but it's the wrong symbol -- we need to
handle this as well as the lookup-by-pc-finds-no-symbol case.

The most obvious fallout from this regression was that lldb on
Mac OS X couldn't backtrace past __assert_rtn() which tail-calls
abort().  e.g.

(lldb) bt
* thread #1: tid = 0x5d6ea1, 0x00007fff8ee80866 libsystem_kernel.dylib`__pthread_kill + 10, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
  * frame #0: 0x00007fff8ee80866 libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x00007fff8eb5835c libsystem_pthread.dylib`pthread_kill + 92
    frame #2: 0x00007fff8852ab1a libsystem_c.dylib`abort + 125
    frame #3: 0x00007fff884f49bf libsystem_c.dylib`__assert_rtn + 321
    frame #4: 0x0000000100000f2c a.out`main + 124

(lldb) dis -c 3 -s 0x7fff884f49b3
libsystem_c.dylib`__assert_rtn + 309:
   0x7fff884f49b3:  movq   %rax, -0x11b96242(%rip)   ; gCRAnnotations + 8
   0x7fff884f49ba:  callq  0x7fff8854fd2c            ; symbol stub for: abort

libsystem_c.dylib`basename:
   0x7fff884f49bf:  pushq  %rbp
(lldb)

in this case, __assert_rtn() is immediately followed by basename() and
the changes in r190812 didn't back up the pc value to get the correct
function name / unwind info.

<rdar://problem/15367233>