[LLDB] Make internal shell the default for running LLDB lit tests.

This patch updates the lld lit test config to use the internal shell by
default. This has some performance advantages (~10-15%) and also
produces nicer failure output. It also updates the two LLDB tests to
not require shell (so that they run under the internal shell), after
first verifying that they run and pass using the internal shell; and it
fixes one test that was not passing under the internal shell.

Author: cmtice

lldb/test/Shell/Process/Optimization.test

@@ -1,5 +1,5 @@
 Test warnings.
-REQUIRES: shell, system-darwin
+REQUIRES: system-darwin
 RUN: %clang_host -O3 %S/Inputs/true.c -std=c99 -g -o %t.exe
 RUN: %lldb -o "b main" -o r -o q -b %t.exe 2>&1 | FileCheck %s
 

lldb/test/Shell/Process/UnsupportedLanguage.test

@@ -1,7 +1,5 @@
 Test unsupported language warning
 
-REQUIRES: shell
-
 RUN: %clang_host %S/Inputs/true.c -std=c99 -g -c -S -emit-llvm -o - \
 RUN:   | sed -e 's/DW_LANG_C99/DW_LANG_Mips_Assembler/g' >%t.ll
 RUN: %clang_host %t.ll -g -o %t.exe

lldb/test/Shell/SymbolFile/DWARF/dwo-missing-error.test

@@ -11,12 +11,12 @@
 # "a.out-dwo-missing-error.dwo".
 # RUN: rm -rf %t.compdir/
 # RUN: mkdir -p %t.compdir/a/b/
-# RUN: cd %t.compdir/a/b/
+# RUN: pushd %t.compdir/a/b/
 # RUN: %clang_host %S/Inputs/dwo-missing-error.c -glldb -gdwarf-5 \
 # RUN:     -gsplit-dwarf -fdebug-prefix-map=%t.compdir=. -o a.out
 # RUN: rm *.dwo
 # RUN: %lldb a.out -s %s -o exit 2>&1 | FileCheck %s 
-# RUN: cd -
+# RUN: popd
 
 # Test the error message with an absolute DW_AT_comp_dir and DW_AT_dwo_name.
 # RUN: rm -rf %t.compdir/

lldb/test/lit.cfg.py

@@ -13,3 +13,15 @@
 config.name = "lldb"
 config.test_source_root = os.path.dirname(__file__)
 config.test_exec_root = os.path.join(config.lldb_obj_root, "test")
+
+# We prefer the lit internal shell which provides a better user experience on
+# failures and is faster unless the user explicitly disables it with
+# LIT_USE_INTERNAL_SHELL=0 env var.
+
+use_lit_shell = True
+lit_shell_env = os.environ.get("LIT_USE_INTERNAL_SHELL")
+if lit_shell_env:
+    use_lit_shell = lit.util.pythonize_bool(lit_shell_env)
+
+if use_lit_shell:
+    os.environ["LIT_USE_INTERNAL_SHELL"] = "1"