Each input file is encoded as a separate compile unit in LLVM debugging

 Each input file is encoded as a separate compile unit in LLVM debugging
information output. However, many target specific tool chains prefer to encode
only one compile unit in an object file. In this situation, the LLVM code
generator will include  debugging information entities in the compile unit
that is marked as main compile unit. The code generator accepts maximum one main
compile unit per module. If a module does not contain any main compile unit
then the code generator will emit multiple compile units in the output object
file.

[Part 1]

Update DebugInfo APIs to accept optional boolean value while creating DICompileUnit  to mark the unit as "main" unit. By defaults all units are considered  non-main.  Update SourceLevelDebugging.html to document "main" compile unit.

Update DebugInfo APIs to not accept and encode separate source file/directory entries while creating various llvm.dbg.* entities. There was a recent, yet to be documented, change to include this additional information so no documentation changes are required here.

Update DwarfDebug to handle "main" compile unit. If "main" compile unit is seen then all DIEs are inserted into "main" compile unit. All other compile units are used to find source location for llvm.dbg.* values. If there is not any "main" compile unit then create unique compile unit DIEs for each llvm.dbg.compile_unit.

[Part 2]

Create separate llvm.dbg.compile_unit for each input file. Mark compile unit create for main_input_filename as "main" compile unit. Use appropriate compile unit, based on source location information collected from the tree node, while creating llvm.dbg.* values using DebugInfo APIs.

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This is Part 1.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63400 91177308-0d34-0410-b5e6-96231b3b80d8