[clang] [test] Use lit Syntax for Environment Variables in Clang subproject (#102647)

This patch updates the clang tests by replacing shell command
substitutions with lit-compatible syntax for setting and referencing
environment variables. Specifically, the use of shell-style variable
substitution (e.g., `DEFAULT_TRIPLE=`and `EXPECTED_RESOURCE_DIR=`) has
been replaced with `env` and `%{env}` to align with lit's internal shell
requirements. These changes ensure that environment variables are
properly set and accessed within the lit environment.

When using the lit internal shell with the command
`LIT_USE_INTERNAL_SHELL=1 ninja check-clang`, one common error
encountered is:
```
FAIL: Clang :: Driver/program-path-priority.c (19 of 20640)
******************** TEST 'Clang :: Driver/program-path-priority.c' FAILED ********************
Exit Code: 127

Command Output (stdout):
--
# RUN: at line 90
DEFAULT_TRIPLE=`/usr/local/google/home/harinidonthula/llvm-project/build/tools/clang/test/Driver/Output/program-path-priority.c.tmp/clang --version | grep "Target:" | cut -d ' ' -f2`
# executed command: 'DEFAULT_TRIPLE=`/usr/local/google/home/harinidonthula/llvm-project/build/tools/clang/test/Driver/Output/program-path-priority.c.tmp/clang' --version
# .---command stderr------------
# | 'DEFAULT_TRIPLE=`/usr/local/google/home/harinidonthula/llvm-project/build/tools/clang/test/Driver/Output/program-path-priority.c.tmp/clang': command not found
# `-----------------------------
# error: command failed with exit status: 127
```
To fix this issue, the patch replaces traditional shell substitutions
with lit's environment variable handling, ensuring compatibility with
the lit internal shell framework. This update applies to both the
handling of the `DEFAULT_TRIPLE` and `EXPECTED_RESOURCE_DIR` variables,
allowing the tests to pass when using the lit internal shell.
The patch also adds `env` to the `PWD` variable setting in the following
command to ensure the environment variable is correctly set within the
lit internal shell:
```
// RUN: %if system-linux %{ env PWD=/proc/self/cwd %clang -### -c --coverage %s -o foo/bar.o 2>&1 | FileCheck --check-prefix=PWD %s %}
```
fixes: #102395
[link to
RFC](https://discourse.llvm.org/t/rfc-enabling-the-lit-internal-shell-by-default/80179)

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