[lldb] Enable using out-of-tree dwps

Previously LLDB required the DWP file 
to be located next to the executable file.
This diff uses the helper function 
Symbols::LocateExecutableSymbolFile to search for 
DWP files in the standard locations for debug symbols.

Test plan:

Build a toy test example:
main.cpp
clang -gsplit-dwarf -g -O0 main.cpp -o main.exe
llvm-dwp -e main.exe -o main.exe.dwp
mkdir -p debug_symbols
mv main.exe.dwp debug_symbols/main.exe.dwp

Run lldb:
lldb
settings set target.debug-file-search-paths ./debug_symbols
file ./main.exe
br set --name f
run

Check that debugging works:
setting breakpoints, printing local variables.

Differential revision: https://reviews.llvm.org/D38568

llvm-svn: 315387