teach clang to install the inline asm diagnostic handler,

allowing backend errors to be mapped through clang's 
diagnostics subsystem, including the backend location info.

We now get:

$ clang asm.c -c -o t.o -integrated-as
<inline asm>:1:2: error: unrecognized instruction
        abc incl    %eax
        ^
1 diagnostic generated.

With colors, and correct "# diagnostics generated".

llvm-svn: 100543