Derive better name for methods with external method tables.

[maleadt]

Fixes JuliaGPU/GPUCompiler.jl#229:

Base.Experimental.@MethodTable(my_method_table)
GPUCompiler.method_table(...) = my_method_table
Base.Experimental.@overlay my_method_table @noinline Base.sin(x::Float32) = x+1

kernel(x) = sin(x)

Before:

; Function Attrs: noinline
define internal fastcc float @julia_my_method_table_519(float %0) unnamed_addr #0 !dbg !36 {
top:
  call void @llvm.dbg.value(metadata float %0, metadata !43, metadata !DIExpression()), !dbg !44
  %1 = fadd float %0, 1.000000e+00, !dbg !45
  ret float %1, !dbg !52
}

define float @julia_kernel_516(float %0) local_unnamed_addr #1 !dbg !53 {
top:
  call void @llvm.dbg.value(metadata float %0, metadata !59, metadata !DIExpression()), !dbg !60
  %1 = call fastcc float @julia_my_method_table_519(float %0) #1, !dbg !61
  ret float %1, !dbg !61
}

After

; Function Attrs: noinline
define internal fastcc float @julia__sin_519(float %0) unnamed_addr #0 !dbg !36 {
top:
  %1 = fadd float %0, 1.000000e+00, !dbg !38
  ret float %1, !dbg !44
}

define float @julia_kernel_516(float %0) local_unnamed_addr #1 !dbg !45 {
top:
  %1 = call fastcc float @julia__sin_519(float %0) #1, !dbg !46
  ret float %1, !dbg !46
}