Switch a bunch of Linker tests from using elaborate echo productions to

just provide and reference separate input files from an Inputs
subdirectory. This pattern works very well in the Clang tree and is
easier to understand in my opinion. It also has fewer limitations and
will remove one particularly annoying use of TCL-style {} quoting from
the testsuite.

Also teach the LLVM lit configuration to avoid recursing into 'Inputs'
subdirectories. This wasn't required for the previous 'Inputs'
subdirectories used due to fortuitous suffix patterns.

This is the first step to completely removing support for TCL-style tests.

llvm-svn: 159520