Do not specify the compilation directory for LLVM modules

In DWARF, the final source code path as determined by the debugger
consists of three parts, recorded in .debug_line:
1) The compilation directory (for a compile unit).
2) The "include directory".
3) The file name.

If the "include directory" is relative (i. e. does not start with 'C:'
or '/'), the final path is going to be the concatenation of all three.

We don't control the "include directory" as it comes directly from
the managed PDBs, and we can't even know if it's truly relative or
an absolute path that just looks like a relative one (e. g. a URL).

Concatenating such an absolute path with any compilation directory
we might have is going to be wrong, so this change stops emitting it.

Author: SingleAccretion

src/coreclr/jit/llvmdebuginfo.cpp

@@ -268,21 +268,18 @@ class TypeDebugInfoModule
     DICompileUnit* InitializeCompileUnit()
     {
         StringRef path = m_context->Module.getName();
-        StringRef dir;
         StringRef name;
         size_t dirEnd = path.find_last_of("\\/");
         if (dirEnd != StringRef::npos)
         {
-            dir = path.take_front(dirEnd);
             name = path.substr(dirEnd + 1);
         }
         else
         {
-            dir = "";
             name = path;
         }
 
-        DIFile* debugFile = m_diBuilder.createFile(name, dir);
+        DIFile* debugFile = m_diBuilder.createFile(name, "");
         return m_diBuilder.createCompileUnit(DW_LANG_C_plus_plus, debugFile, "ILC", false, "", 1, "",
             DICompileUnit::FullDebug, 0, false);
     }