[PECOFF] Create __ImageBase symbol.

__ImageBase is a symbol having 4 byte integer equal to the image base address
of the resultant executable. The linker is expected to create the symbol as if
it were read from a file.

In order to emit the symbol contents only when the symbol is actually
referenced, we created a pseudo library file to wrap the linker generated
symbol. The library file member is emitted to the output only when the member
is actually referenced, which is suitable for our purpose.

llvm-svn: 188052

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