Replace dashes with underscores in llvm-patches (#2211)

Dashes in bazel package names in particular can cause problems (which renaming `llvm-patches` to `llvm_patches` will fix for me). But we've generally named files with underscores, so there's also consistency.

.pre-commit-config.yaml

@@ -179,7 +179,7 @@ repos:
               .bazelversion|
               compile_flags.txt|
               third_party/.*|
-              bazel/llvm-patches/.*\.patch|
+              bazel/llvm_patches/.*\.patch|
               .*\.def|
               .*\.svg|
               .*/fuzzer_corpus/.*|

WORKSPACE

@@ -108,8 +108,8 @@ http_archive(
     build_file_content = "# empty",
     patch_args = ["-p1"],
     patches = [
-        "@carbon//bazel/llvm-patches:0001-Patch-for-mallinfo2-when-using-Bazel-build-system.patch",
-        "@carbon//bazel/llvm-patches:0002-Added-Bazel-build-for-compiler-rt-fuzzer.patch",
+        "@carbon//bazel/llvm_patches:0001_Patch_for_mallinfo2_when_using_Bazel_build_system.patch",
+        "@carbon//bazel/llvm_patches:0002_Added_Bazel_build_for_compiler_rt_fuzzer.patch",
     ],
     sha256 = "0a3929c5f2fe756820277be7b10e95f7480e7cb7f297ec574d3e9ddeac9068d7",
     strip_prefix = "llvm-project-%s" % llvm_version,