[LLDB][NativePDB] Convert backslash to slash when creating CU and filter out...

[LLDB][NativePDB] Convert backslash to slash when creating CU and filter out CU with no function in ResolveSymbolContext.

On Windows, when compile with -fdebug-compilation-dir which contains slash, the source file path in PDB will look like "../tmp\file.cc" because the path separator used is determined by target machine. Converting backslash to slash helps lldb to find the CU in ResolveSymbolContext.

We want to filter out CU with no function in ResolveSymbolContext as a cpp file will have two debug info modules in PDB if built with thinlto and one of them is a skeleton with no function debug info.