Update lit config for ld.lld command

[rdhindsa]

Summary

1. Updated lit config for ld.lld command, for it to match "ld\.lld" instead of trying to match respective regex. (It was able to work with ld-lld and ld1lld as well)
2. Fixed the lld test using ld-lld command

[rupprecht]

Since this spans the llvm+lld repositories, I think you may need to do this as two separate commits. It would be good to commit the lld test first to avoid any breakage caused by the lit change.

[rdhindsa]

Thanks for the review! I will check them in as two separate commits.

lld/test/ELF/arm-exidx-add-missing.s

 // REQUIRES: arm
 // RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi --arm-add-build-attributes %s -o %t
-// RUN: ld-lld %t --no-merge-exidx-entries -o %t2
+// RUN: ld.lld %t --no-merge-exidx-entries -o %t2
 // RUN: llvm-objdump -s %t2 | FileCheck %s
-// RUN: ld-lld %t -o %t3
+// RUN: ld.lld %t -o %t3
 // RUN: llvm-objdump -s %t3 | FileCheck %s -check-prefix=CHECK-MERGE
 
 // The ARM.exidx section is a table of 8-byte entries of the form:
 // | PREL31 Relocation to start of function | Unwinding information |
 // The range of addresses covered by the table entry is terminated by the
 // next table entry. This means that an executable section without a .ARM.exidx
 // section does not terminate the range of addresses. To fix this the linker
 // synthesises an EXIDX_CANTUNWIND entry for each section wihout a .ARM.exidx

llvm/utils/lit/lit/llvm/config.py

         ld_lld = self.use_llvm_tool('ld.lld', required=required)
         lld_link = self.use_llvm_tool('lld-link', required=required)
         ld64_lld = self.use_llvm_tool('ld64.lld', required=required)
         wasm_ld = self.use_llvm_tool('wasm-ld', required=required)
 
         was_found = ld_lld and lld_link and ld64_lld and wasm_ld
         tool_substitutions = []
         if ld_lld:
-            tool_substitutions.append(ToolSubst('ld.lld', command=ld_lld))
+            tool_substitutions.append(ToolSubst('ld\.lld', command=ld_lld))
         if lld_link:
             tool_substitutions.append(ToolSubst('lld-link', command=lld_link))
         if ld64_lld:
-            tool_substitutions.append(ToolSubst('ld64.lld', command=ld64_lld))
+            tool_substitutions.append(ToolSubst('ld64\.lld', command=ld64_lld))
         if wasm_ld:
             tool_substitutions.append(ToolSubst('wasm-ld', command=wasm_ld))
         self.add_tool_substitutions(tool_substitutions)
         return was_found