Initial dsymutil tool commit.

The goal of this tool is to replicate Darwin's dsymutil functionality
based on LLVM. dsymutil is a DWARF linker. Darwin's linker (ld64) does
not link the debug information, it leaves it in the object files in
relocatable form, but embbeds a `debug map` into the executable that
describes where to find the debug information and how to relocate it.
When releasing/archiving a binary, dsymutil is called to link all the DWARF
information into a `dsym bundle` that can distributed/stored along with
the binary.

With this commit, the LLVM based dsymutil is just able to parse the STABS
debug maps embedded by ld64 in linked binaries (and not all of them, for
example archives aren't supported yet).

Note that the tool directory is called dsymutil, but the executable is
currently called llvm-dsymutil. This discrepancy will disappear once the
tool will be feature complete. At this point the executable will be renamed
to dsymutil, but until then you do not want it to override the system one.

    Differential Revision: http://reviews.llvm.org/D6242

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224134 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused feature. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224135 91177308-0d34-0410-b5e6-96231b3b80d8
Pass a FD to resise_file and add a testcase.

I will add a real use in another commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224136 91177308-0d34-0410-b5e6-96231b3b80d8
Move the resize file feature from mapped_file_region to the only user.

This removes a duplicated stat on every file that llvm-ar looks at.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224138 91177308-0d34-0410-b5e6-96231b3b80d8