Add support for having multiple sections with the same name and comdat.

Add support for having multiple sections with the same name and comdat.

Using this in combination with -ffunction-sections allows LLVM to output a .o
file with mulitple sections named .text. This saves space by avoiding long
unique names of the form .text.<C++ mangled name>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228980 91177308-0d34-0410-b5e6-96231b3b80d8