Three files were duplicated in the same source_set for LLVM 10, but with
different case:

"llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp",
"llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp",
"llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp",

"llvm/lib/DebugInfo/DWARF/DWARFCompileUnit.cpp",
"llvm/lib/DebugInfo/DWARF/DWARFExpression.cpp",
"llvm/lib/DebugInfo/DWARF/DWARFCompileUnit.cpp",

GN/Ninja doesn't like this, and it only failed on Mac for some reason
(See https://bugs.chromium.org/p/gn/issues/detail?id=180).
Split out the DebugInfo files into a separate source_set to fix this.

Bug: b/152339534
Change-Id: I294d48945ac5b47971b8387948bbd05767f7095c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46568Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>