[LLD][ELF] Add test cases to improve code coverage of Thunks [NFC]

[peter.smith]

Summary

Add two test cases to improve the code coverage of ThunkSection creation when there are no existing ThunkSections in range. There are two test cases, one where a new section can be created and another to trigger the "InputSection too large for range extension thunk" error message. A recent code coverage report showed that this section of code wasn't covered by a test case.

[ruiu]

LGTM

lld/trunk/test/ELF/arm-thunk-nosuitable.s

+// RUN: llvm-mc %s --arm-add-build-attributes --triple=armv7a-linux-gnueabihf --filetype=obj -o %t.o
+// RUN: ld.lld %t.o -o %t
+// RUN: llvm-objdump -triple=thumbv7a-linux-gnueabihf -d -start-address=2166784 -stop-address=2166794 %t | FileCheck %s
+// REQUIRES: ARM
+
+        // Create a conditional branch too far away from a precreated thunk
+        // section. This will need a thunk section created within range.
+        .syntax unified
+        .thumb
+
+        .section .text.0, "ax", %progbits
+        .space 2 * 1024 * 1024
+        .globl _start
+        .type _start, %function
+_start:
+        // Range of +/- 1 Megabyte, new ThunkSection will need creating after
+        // .text.1
+        beq.w target
+        .section .text.1, "ax", %progbits
+        bx lr
+
+// CHECK: _start:
+// CHECK-NEXT:   211000:        00 f0 00 80     beq.w   #0
+// CHECK: __Thumbv7ABSLongThunk_target:
+// CHECK-NEXT:   211004:        00 f0 01 90     b.w     #12582914
+// CHECK: $t.1:
+// CHECK-NEXT:   211008:        70 47   bx      lr
+
+        .section .text.2, "ax", %progbits
+        .space 12 * 1024 * 1024
+        .globl target
+        .type target, %function
+target: bx lr

lld/trunk/test/ELF/arm-thunk-section-too-large.s

+// RUN: llvm-mc %s -triple=armv7a-linux-gnueabihf -arm-add-build-attributes -filetype=obj -o %t.o
+// RUN: not ld.lld %t.o -o %t 2>&1 | FileCheck %s
+// REQUIRES: ARM
+
+// CHECK: InputSection too large for range extension thunk
+        .syntax unified
+        .thumb
+        .text
+        .globl _start
+        .type _start, %function
+_start:
+        .space 2 * 1024 * 1024
+        // conditional branch has range of 1 Mb expect error as we can't place
+        // a thunk in range of the branch.
+        beq target
+        .space 2 * 1024 * 1024
+
+        .section .text.2, "ax", %progbits
+        .globl target
+        .type target, %function
+target: bx lr